Probably eating too much, too fast.
My father-in-law was in hospital recently, for 3 weeks, and I looked after his cat. For 3 weeks, the cat was no trouble at all. The day after he came back, the cat was sick 5 times in one day. On the same food. The only difference seemed to be that I fed the cat a small handful of food twice a day, which the cat eat up, and he leaves bowls full of food down all day.
So either the cat was eating itself sick, or the food was going off whilst left out (though it was always dry food).