They drank too much too quickly and overfilled their crop so it started coming back up. I've had this happen before. It's scary, but it's nothing to worry about.
In other, sadder, news, the **** ravens got my duckling!!!! I had let the cemanis out to free range and the ravens went through the pop door into their run, then through the pop door into the coop, then got into the brooder and took my duckling out, into the run, and ate her

All that was left was feathers.
And then they took two of the guinea keets that were in with her and were working on a third when I heard an unholy ruckus in the yard and went out to investigate. They had made an error and taken the third keet out of the coop and run and into the yard (or it took itself out there trying to get away), where its distress cries made my male guinea and my two juveniles from this year very angry. I went out to see the keet running for its life and the others pummeling the crap out of two ravens, who weren't backing down (until they caught site of me). So that keet got lucky and made it. It and the surviving keet were relocated to my Eglu coop which is totally raven proof.
So these ravens are likely to come down with a severe case of lead poisoning if I can get them. If not, they're liable to come down with a case of regular poisoning. Hope they like chicken with a side of rat poison.