Nice find! So usually when a wave comes in we give it a bit of an initiative boost so that they are able to get some turns in before the player does their thing. I think the UI didn't clean itself up properly after you cleared the prior wave with Party Surprise (because it's trigger can happen in between turns, any source of damage is enough to set it off) which might have resulted in this scenario. Will note this for the next patch :)
Oh, wait, okay, after another test, I am pretty sure I understand what's going on. Unsure why the Bomb skill triggers it like this, but what it looks like to me is that it's sending the next two waves at the same time. They're just layered on top of each other. Some extra testing would not be unwarranted, but I definitely saw this happen at least once.