Google, at-will employment.
Obviously we do not have all the facts here; nor have we read your CBA. But in 49 out of 50 states, employment can be severed at any time by either party for any reason except one specifically prohibited by law, and even in the 50th state I'm not convinced that refusal to renew an expired contract would qualify as an illegal reason. It's definitely not an illegal reason in the 49 so-called at will states.
That's why your only recourse is through your union. There simply is not a law in NJ, or any other state for that matter, that requires that they renew your contract when it is up. That's why it's not illegal; because there isn't a law that says they can't do what they're doing.