New tank here, have some random questions.

[BarretOblivion]

Tankbusters... you get hit and watch your health, if its a big chunk or you just die... that's a tankbuster. Most fights currently have a list of tankbusters on the internet so if you are doing some sort of end game content a quick look will give you what you need.
The tank stance turn off happens when you sync down for dungeons/ other content. Its just a wierd thing.

[TheForce]

Regarding TB's, there's not really a "one-size-fits-all" way of knowing if an incoming attack will be a Tank Buster aside from just memorizing the name of it, however, the vast majority of them require a cast, so be sure to always keep an eye on the boss's cast bar, and also the name of the attack being cast.

But there are some TB's that don't require the boss to cast them, and they come out very quickly and seemingly randomly, don't fret though, most bosses follow a pattern in the attacks they cast, so the moment you see the the attack that precedes the TB, start preparing and you'll be good (The second boss of Akadaemia is like this, Lash is the TB but there is no cast bar for it).

[Conando]

It gets shut off by level sync. It’s always been a thing for some bizarre reason. So just remember, if your level is synced down tank stance will start shut off.

[3c-33]

There's some pretty big inconsistencies with tanking from dungeon to dungeon as far as tankbuster damage goes and when they show up or if they're even worth mitigating. in the level 50-60 dungeons, if you're adequately geared, the damage that comes from them is laughable for the most part

What I've found is that there's generally 3 kinds of tankbusters that you'll see

1. completely random busters, honestly no way of knowing when they'll do it. In particular for this kind I can only think of dohn mheg's second boss (unless I'm dumb but I swear I can never tell when it's going to come out so I end up getting chunked for 3/4 of my hp every time lol) and I'm sure there's others I can't remember right now

2. Stormblood starts introducing busters that tend to always occur after a set of mechanics, starting right away with sirensong's final boss. Every time the boss casts "morbid march/retreat," an instant cast buster almost immediately follows it, which is a fairly recognizable enough pattern when you've run it enough times to notice. The final bosses of shisui and bardam's mettle follow this pattern too, as do others I'm sure I can't remember off the top of my head

3. The ones you can actually see the cast for. I atleast noticed for shadowbringers, nearly every buster I saw casted sounded like one while I was running them blind, which was something I enjoyed, names that clearly indicate what's about to happen (The tickler, headcrusher, crippling blow, stonefist, rend, ripper fang, etc) Of course there's also plenty of the more odd ones you can't really tell that I still can't remember for the life of me (like most of mt gulg honestly)

[Tint]

1. completely random busters, honestly no way of knowing when they'll do it. In particular for this kind I can only think of dohn mheg's second boss (unless I'm dumb but I swear I can never tell when it's going to come out so I end up getting chunked for 3/4 of my hp every time lol) and I'm sure there's others I can't remember right now

That's not for a specific boss, but I have heared of tanks who count the auto attacks the boss does and know that after maybe every 8 auto attacks comes the tank buster or so. My concentration is not good enough for that... >.>

[ilione]

if it sounds like ouch (e.g. stonecrusher, ravenous assault), it's probably a buster. if it also sounds spooky, it might be a magical buster (shadowflame). sometimes se is a troll and makes magical busters that sound physical (ravenous assault) but you don't have to worry about that much unless you play drk or to an extent gnb

occasionally you'll have bosses with few to no busters per say, but heavy autos that chunk you as well, then you just have to memorize when they go out

[Ryphen]

1. completely random busters, honestly no way of knowing when they'll do it. In particular for this kind I can only think of dohn mheg's second boss (unless I'm dumb but I swear I can never tell when it's going to come out so I end up getting chunked for 3/4 of my hp every time lol) and I'm sure there's others I can't remember right now

That boss's TB isn't as random as you think. It's called Rake and it follows the rules of your second example. The first one happens a few seconds after you engage him. So if you lob to pull, then run through him while hitting your next GCD, by the time he spins around to face you and not the rest of the party, he's about ready to nail you. He does another one after a specific set of mechanics but I can't remember which off the top of my head.

[Windwalker]

First, why does tank stance turn off at the beginning of dungeons randomly? I swear that trips me up a good 30% of the time; run in, pull a bunch of crap, notice that enmity is all over the place and "DAMNIT! AGAIN!?" I'm sure I'll get used to it, but just... why?

Second, how do you recognize tank busters? Is there some kind of specific tell or do I need to just pop sheltron every time it looks the boss is winding up for something?

Think that's it. Tanking is actually pretty fun; I like being the one in the group who can fight everything head on. My sense of direction sucks, though.

Tank stance turns off when you enter lower level dungeons or leave them to enter higher level dungeons, also in many cases i forgot putting the stance back on while checking my other jobs waiting for dungeon queue.

[Iscah]

It's not just tank stance that gets turned off when you level-sync - it removes any similar status buffs from other classes and resets built-up gauges to zero.

[3c-33]

That boss's TB isn't as random as you think. It's called Rake and it follows the rules of your second example. The first one happens a few seconds after you engage him. So if you lob to pull, then run through him while hitting your next GCD, by the time he spins around to face you and not the rest of the party, he's about ready to nail you. He does another one after a specific set of mechanics but I can't remember which off the top of my head.

idk maybe im dumb ): i know i've had several moments where i keep using cooldowns in anticiapation of it only for it to never come, then i have nothing when it does hit lmao