Its funny that I read this this morning after having 3 students in a row yesterday who all had a similar problem. Every time the source of the problem was their heels. If you don't anchor your heel your leg is more likely to slip back, heels up which will make it so much easier to drop your stirrup. It also often sends your upper body forward a little which doesnt help.
I wouldnt recommend leaning to that side to compensate for that. If you are going to lean then you will end up off centre therefore off balance and this doesnt help you at all. Work on keeping your heels down. A good way to do this is by 2 point seat in walk and trot. Your hips, knees and ankles should be like springs driving the pressure down the back of your leg to your heels. Be careful not to overdo it and push them down too far though :)
Having said all that, some people do have different length legs so I would check that. It might be good to get some pics or videos when you are riding and then it might be easier to tell.