Even if a tank isn't popping CDs constantly, they shouldn't be that squishy, especially not in leveling dungeons. Leveling dungeons are designed to be run even if you're a bit under-geared, you should still be fine for the most part, except for a few outliers (Bardam's Mettle for one, some of the enemies in there hit extraordinarily hard and it catches newbie tanks and healers off-guard).
I wouldn't blame this on CDs unless they are recklessly pulling multiple packs and THEN not using CDs.
It usually falls to one of the following:
1). Poor Gear Choices: They might be using DPS accessories and their mitigation stats are terrible.
2). No Tank Stance: Tanks love going DPS on huge packs, dunno why, because when you throw DPS stance on and proceed to pull 20 mobs you're going to die unless you've got insane gear and/or you're going to lose threat to the DPS that are AoEing like crazy, OR you are going to gobble up all the healer's GCD/Casts just to keep you alive. Dunno how many times as RDM, I see a tank pull, use an AoE and my second Scatter makes me the new tank. That Should. Not. Be. Happening. I look up and oh. He's got his DPS stance on. Lovely. I, as the DPS, have to stop my DPS so I don't die so that he can do more DPS. Riiight. That, and sometimes, for the first couple pulls even I myself forget to turn Stance on at all. I wish Duty Finder would stop cancelling all of my stances and buffs. The ONLY time that should happen, is if I go into a lower level dungeon, like Sastasha or something that my level is too low for my stance. That, or give stances from the get-go so we can stop cancelling stances every. freaking. time. I enter the dungeon.
3). Just undergeared in general: If a tank's HP is barely above your own, he's probably at minimum I-level for the dungeon. A tank can do a dungeon like this, but he needs to single-pull and you need to pay more attention than usual.