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).
It's not limited to specific mobs per zone - the only time you'll get heavy status in an aggressive zone whilst not mounted is if the mobs actually have a heavy inducing attack. (Which for the most part is just monster skills, to my recollection)
That said the Heavy is more annoying than it should be, and should be toned down a smidge.