Callie likes to hear a pred burp from inside the pred and she likes people to hear her burp from inside her. Her very active sex life puts her in contact with a lot of potential preds and a lot of potential prey and she takes advantage of meeting both. Callie knows what she likes and she goes and does it.

Callie is the only switch pred/prey on the beach because Jeff nor Beachcomber aren't sure the "humans always reform" rule applies to them as semi-ferals. In Callie's place they would probably have had the same worry about anthros and not gone and stuck their head in anyone's mouth, but by the time anyone raised that point with Callie she'd already been through a dozen digestive tracts. Obviously it all worked out.