Quote Originally Posted by SetoKaiba View Post
The heavy debuff in question comes from any mob in game, as long as you are mounted you have an X% chance to be inflicted with heavy on hit (100% in PVP).
That explains it... one time I was passing by an archer in Zahar'ak and every hit refreshed the Heavy debuff. Every. Single. Hit. That mob was so good at RNG.

I noticed that higher level mobs tend to inflict Heavy more though? As I did not notice a significant infliction of Heavy from other aggressive mobs while I was mounted and using lower leveled classes.