Dipper had come to the mystery shack earlier, he suspected robbie was a zombie, so he was going to prove it to Wendy, 

And sometime after that, marry her. Dipper headed in " Wendy!" He called out he heard a crunching noise- what if robbie Had killed wendy? He ran forwards.... Only to see Wendy, he mouth buried in a raccoon, devouring its flesh. Dipper stared in utter shock.... Then he fainted