Possible reasons for a bracket coming off are: a wet tooth surface when the bracket was fitted, eating or biting into hard food, contact between the upper teeth and the lower brackets when eating, and clenching the teeth. The most common reason for a bracket coming off at the start of treatment is a poor bond between the adhesive and the tooth when it was fitted. If this happens, of course, it is not the patient's fault.
Sometimes, while we are getting used to the diet that braces require, we may eat foods that aren't very suitable for the appliance. This is a process of adapting to something new in the mouth. At first the patient is very anxious and eats only soft foods, mostly puréed. Over time they find that the braces are firmly in place and start allowing themselves slightly harder foods. Some patients allow themselves foods that are far too hard, until a bracket comes off. Then the patient works out what not to eat and how careful to be to avoid similar problems.
Contact between the upper teeth and the lower brackets is more common with a deep bite. We usually prevent such contacts by placing materials in the mouth that stop the teeth from closing fully. If for some reason these materials don't block the contact well enough, the lower bracket comes off again through no fault of the patient. Once we correct the problem, this shouldn't happen again.
This answer is for general information. Your own case needs an examination by an orthodontist.