Term for the error in the reason is because
The reason is because (as previously queried on ELU in that link) is a form of redundancy (perhaps tautology, pleonasm), because the reason and because both convey the sense of why, what causes something to be the case.
Pedants will thus say you should use either the reason is X or [it] is because X, but not both. But many native speakers routinely ignore such advice - it's certainly not universally considered an "error".