I just checked your armory, your Combat Table Coverage is 99.72%. The cap is 102.4% but you're not wearing any gloves, so it's quite likely that you are indeed capped, so that is probably not the issues.
The obvious answer would be "They just hit hard.." It's probably not what you want to hear, I know! What I like to do is just chain cooldowns (but save them for hard hitting abilities that you know are coming - Bigwigs/DBM is your best friend!). If you're having trouble with fights where there is a lot of spelldamage, try glyphing divine protection and getting the trinket from Tol'Barad. 2 extra cooldowns for magic damage right there. Lastly, use your cooldowns before you take damage, not when you're at 20% health. It sounds easy but a lot of people will have the reflex to really use them as panic buttons, they go "OH SNAP I JUST GOT HIT FOR 150K) then hit all their cd's at once - not the best thing to do.
While I haven't said anything you won't find in the tankspot/elitist jerks compendium I hope this will have helped at least a bit.. Just hope your healers are decent and remember to face the boss