Same situation with Striders back in Zangermarsh. Had over 490 def rating and got crit by a level 64 while level 70.
At 80, its no supprise to see the same situation pop up with certain mobs. If you have 540 defence, the only way you will be crit is through a debuff reducing your defence, or by a mob having an increased chance to crit, or a buff increasing its chance to crit. Its very unlikely that it will happen on a boss mob.
One other thing that use to have to be considered, was a talented judgement of the crusader being spell reflected. Man that use to be fun times.
_________________
"Good tankadins know all the fundamental rules of tanking
Great tankadins know when to break these rules" -Passionario