Tank got crit on Prince fight

As the title states, I was tanking Prince in Karazhan last night and everything was going smooth as ussual. Tanked him several times already and my hp never drops below 60% thanks to being 2% away from uncrushable and some awesome guild healers. Then during P2 with like 3% left til P3 i get 1 shots at full hp the instant I used my Moroes trinket. I had just got the Moroes trinket this run so I have never used it on Prince before until last night. When I brought up the log, its shows 2 attacks as critical strikes with me being at 492 defense. Not sure how or why. Figured I'd post a screenshot and let someone smarter than me figure it out. Still killed him on the next attempt and the damage was smooth as usual with no random crits and used block trinket instead of Moroes trinket.

http://imgur.com/a/GSjGp Here is the screenshot