I don't know all the details or repercussions of the situation, but as I understand it, non-union voice work is in fact the primary reason for voice actors to go uncredited. (Or on occasion use aliases.)
Yes, many voice actors are indeed SAG members who do non-union work. Working on on such projects is extremely risky for them because it can result disciplinary action, such as heavy fines. Perhaps they can even be ejected from the union, but that's just speculation on my part.