It is unlikely that facebook or google sign up will present the problem.
The more logical reason would be based on some sort of activity in forums or what not (or some other criteria that they programmed into their filtering system)…
This is to filter out people signing up with multiple accounts.
But note that this is just speculation on my side. If you’d like, ask your friend or whoever has this problem to msg the staff directly. They will most likely provide you with an account specific answer.