Percival is such a sweetie that it's hard to imagine him eating people of his own accord, so there's a few ways I tend to imagine it possibly happening:

• It's an accident. Oops, Percival has gotten so big and gluttonous that sometimes he can just swallow people whole without meaning to! Sometimes paired with the idea of them getting shrunken down to tiny size to make it slightly more plausible lol
• Someone else either tricks or forces him into it. The idea of feeders in vore is wildly underrated. Sometimes your fav just needs a little nudge in the right direction...
• The prey actually wants to be eaten. I think he would be cutely flustered, but eventually agree to it.

Also, Percival makes amazing prey for anyone gluttonous to eat such a gigantic man.