BTW: It's very rare that people swallow their tongue (if you don't caunt a wrong guedel-airway insertion),
but a cause could be that the small piece of tissue under the tongue (we in The Netherlands call it the tongue-belt, but do't know how you call it in the US), that connects it to the bottom jaw tears off.
When that occurs, it's usually seen in soccer-matches when a player gets a very hard ball into his face, resulting in the tongue-belt breaking, not during a seizure.