your diet can effect it, you could be walking a bit more and not even realize, you could have boost your metabolism by adding something into your diet or daily routine without realizing it. if youve de-stressed recently that couldve also helped or if you're now having sex and you werent before or you're having more than before that could be the cause. some other things could cause changes in the appearance of butt and thighs, exfoliation, moisturizing or hydration (drinking lots of water). but no your body always has reasons for the things it does, though they can go on without you noticing them.
If your diet or eating habits have changed then that could be the cause. Maybe some stress dropped out of your life so you're able to enjoy it a little more. And oddly enough are you eating differently? Like earlier in the day or at a slower rate? Eating early in the day gets your metabolism working earlier at a better rate. Chewing your food more and slower breaks it down more and makes it easier to break down. Not eating before you go to bed could be it to, maybe a mix of all.
I'm not sure if they can become toned all by themselves but that might be because my definition of toned is something along the lines of 'muscular without being bulky.' I DO believe that your body can become thinner even if you don't work out so that might end up with your butt and thighs looking firmer than they were when they were flabby.