If you double click a window then it tends to maximize. If you are seeing that with a single click then it is possible the double click detection rate is too high. Go to the mouse settings and lower it.
You didn't mention the app but it is also possible that is just how the app behaves. It would be annoying if it did but it sounds like it just started so maybe not.
If the app starts up in maximize mode then change the shortcut setting to have to start normal (and hope it doesn't default to full screen).