Imogen figures, wherever this guy came from, has to be something important. "Everyone through the tunnel!" If he's confused he won't be able to stomp them all down in the enclosed space, and maybe he'll crash into a wall in the process which might give some help. If he doesn't take the tunnel down on their heads.

So best to hedge those bets, Imogen will give everyone a head start and Quash the big guy again with a rush of dark energies.