Couple things...

1. Particularly at 65+, old gear is now far behind dungeon gear. Sometimes tanks try them in level 60 gear (I even had to quit the 69 one once because of such a tank), but...
2. This can happen pre 60 too, especially with newer tanks. And newer healers that aren't in sync'd gear.
Then, finally... 3. The big pull meta. No place in most leveling dungeons but a lot of tanks like to try it anyways. Bonus points if they don't use CDs.

Welcome to the healer life.