Is there a debuff prioritization?

Mobs currently are balanced with a 16 debuff limit, meaning any more than 16 and some will get pushed off as new ones are introduced. There has been a debuff prioritization system in place since Patch 1.10 where certain debuffs are "protected" and kept from being pushed off by lesser debuffs and dots. The protected debuffs include CoS, CoE, CoR, Sunder, Hunter's Mark, etc.

DoTs are NOT protected. Your Corruptions, CoA, and CoD are all susceptible to being pushed off. Certain AOE spells such as Hellfire and Rain of Fire count for an invisible debuff slot, although it doesn't show up under the mob's portrait. The mage spell Arcane Missiles is another spell that does this, which is why you might notice your Corruptions disappearing on Vael after only a few ticks.

NOTE: It has been confirmed that the debuff limit will be raised from 16 to 40 in the expansion.