uef, I watched replay again and the reason was that one of mercies that got shot down first blocked shots from maa trying to hit the mercies behind it with its falling wreck (why don't falling wrecks take damage?)
another reason is that maa often can't turn around and start shooting fast enough to hit the mercies as they come in before they turn into an untargetable swarm, also even with target priorities it is abusable to make it turn to wrong direction before mercies arrive and having no time to turn back before they are already untargetable