It's more about which unions are willing to represent them. Sometimes it makes for strange bedfellows and sometimes it results in inter-union competition, but there are no strict laws on who can represent who as far as I know.

Some unions are more willing to take on an underdog, whereas others only get involved if it feels like a sure thing. Sometimes a union will support a drive if it specifically strengthens their position (like an adjacent industry). And some industries like fast food the big unions feel are a lost cause, so most union activity there is independent.