Why do my sims randomly get caught on fire?

Not exactly my sims but I was following my Sims boyfriend to figure out where he lived or what he does when he leaves or whatever lol And I followed him out to a garden that's near my house. He fell asleep there and woke up only to be caught on fire. He wasn't even near a flammable object lol and this happens a few times more or so and I'm just wondering why does it happen? Also, how can I get costumes for the season of fall? Thanks