What causes relapse of bell's palsy?

Bell palsy relapse: Mostly bad luck. Probably chronic stress and reduced immunity are factors, but it is very difficult to prove. It is well known that some people are prone to repeated episodes of bell's palsy, but this is unusual, and there are no known proven strategies to prevent these relapses.

Other possibilities: There are some diseases that can cause recurrent facial paralysis. Strictly speaking, it then isn't really "bell's palsy". Melkersson-rosenthal syndrome is one, and some other autoimmune diseases can do this.