When a person doesn't swallow correctly, it's called a tongue thrust because the tongue, one of the most powerful muscles in the body, thrusts against the teeth when swallowing, over time forcing them out of alignment.
In most cases, though it's an involuntary behavior, if detected early enough it can be corrected by either therapy or dental appliances. However, if undetected as the source of misalignment, it can easily undo the work of braces.
A tongue thrust can be caused by thumbsucking, by abnormally large tonsils or tongue, and allergies, just to name a few possibilities. There are also different kinds of tongue thrusts. The most common one is an anterior thrust, in which the tongue pushes against the front teeth.