Window in garage keeps opening on its own?

I recently moved out from CA and now live in the SE and it’s been really cold lately (actually snowing today) but I noticed something weird in my garage. It keeps opening just a crack even when I close it several times. It’s a hanging window so don’t know what is causing this. Wife told me she hasn’t touched that window ever. I started thinking someone was coming in just to crack the window open but it has storm windows on it. Anyone know what’s causing this?