To me it seems like it happens because of the low mana cast, once a mob has gone out of mana, the only spell it will cast from that point forward is dispel, as that is the first it can afford.
Also, not totally related to dispel, but spell seems to be a priority basis, ie. certain mobs won't cast a x spell unless it has already landed y spell
Personally I don't mind it. Pick up some items and junk buff properly and you'll find yourself much preferring dispel to a couple of Ice Comets that love to hit thru 140 CR for full damage
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