Freezer door pops open, how can I stop this?

My freezer door pops open just a little bit, each time the main fridge door is closed, just such a small amount it is hard to notice. After a few opening and closing of the main door there is quite a gap in the freezer door. I was wondering why ice was building up inside but now realize it is due to the door being popped open for enough warm are to get in. I at first accused my roommate of not closing the freezer door hard enough, until I noticed it popped open when I closed the main door. It is only a tiny bit each time and it seems that the whoose of air from the lower door closing is opening the freezer door... is there anyway of getting the freezer door to close tighter so it won't pop open? It is a Sears Kenmore freezer on the top model. Thanks!