i had it happen with different chars so far.
Acolyte/ that Scythe attack animation (forgot name, reaping strike? you know the one you deal double dmg on marked enemys).
Primalist/ that elemental hit (with phys/light/ice dmg procs on it)
Sometimes after attacking your character is frozen in place and you cant move until you used an ability.
If i fully clear a map it happens 2-4 times.