That's cause bad tanks/new tanks, don't know how the aggro bars work when looking at their target. If they flash, they'll notice the aggro bars change.

It's just in my experience, a lot of plds run through the pre50 dungeons, and only flash once at the beginning. When they need to treat it like OP and flash a few times to build up solid starting aoe enmity.

I also suspect, that some tanks feel like they need to do dmg exclusively to tank(to my understanding, current WoW is like this), where my sole focus first and foremost is that I'm number one on the enemies list, and dmg can come as long as I won't lose hate.