Put some heavy stuff on the door like soda or water bottles,also wash around the frame where the door gasket touches the fridge and wash the gasket too, there are magnets inside the gasket and if there's anything on the frame it could stop the gasket from making a good seal,how old is the fridge?what shape is the gasket in?if it's torn or lost it's shape it should be replaced.most likely you might just have to get some weight on the door to stop it from popping open