I don't know why she's doing it, but I'm sure she has her reasons and probably won't tell you what they are.....

seriously though, what are the temps currently? If it's not the temps, could just be she needs a change. Smitty's temps are always in the 'good' range, but just recently he took to curling up on top of his cool side hide--like Snoopy! This lasted a couple of weeks and now he's back to sleeping inside them. Temps were no different, but I guess he just needed a change of scenery. I'm sure it's nothing to worry about.