Easy to reproduce... when being attacked by raiders, leave one dead on the floor (i.e don't click on them), when the next raid occurs, wait until they're attacking the door and then click on the dead raider from the previous attack, it will 'cash in' and one of the raiders outside the door will keel over and die. Tap on them and another dies, I haven't tried killing them all, I thought it probably not a good idea to kill off the raiders before they had got in! Anyway, all you need to do is not tap the last raider for each attack and you'll have an easy life.
One thing I was going to test but haven't had the chance yet, what happens if you tap on the dead raider during a deathclaw attack...