When you start casting iron grenades (or I guess other grenades) while standing still and start moving mid-cast client fires UNIT_SPELLCAST_INTERRUPTED falsely.
On the other hand if you start casting it while moving and never stop then cast is detected and shown normally.
I guess that player starting moving is how interrupted cast is detected, but in this particular case it does so incorrectly.
When you start casting iron grenades (or I guess other grenades) while standing still and start moving mid-cast client fires UNIT_SPELLCAST_INTERRUPTED falsely.
On the other hand if you start casting it while moving and never stop then cast is detected and shown normally.
I guess that player starting moving is how interrupted cast is detected, but in this particular case it does so incorrectly.