While I appreciate that you want an answer, under the law they don't have to give an answer. When you are in a probationary period for a union job, that is the same as being an "at will" employee in any other job. Legally, an at will employee can be terminated at any time, with or without cause, and because of that the employer is not legally required to give a reason.
Their reason can literally be that you were in the probationary period and they didn't want to keep you, period. No other reason is legally required.
The only way to force them into perhaps giving a cause is to file an EEOC complaint, alleging discrimination based on race, religion, gender, age or disability. You typically need more than just a termination to make that allegation, like some comments made about one of those factors or being replaced by someone that was in a different category in one of those factors than yourself. The EEOC can investigate the matter and the employer would then be compelled, in defense of the allegation, to give their actual reason for the termination...if they even have one.
It's very possible they just want to cycle through people, treating them as temps and letting them go before they come under the protections of the union CBA. While that's unprofessional, it's not illegal.
Regrettably though, if there is no means of suggesting discrimination and you don't feel that is at the heart of this issue (enough to justify filing a complaint with the EEOC), there is no legal means of forcing them to state why you were let go. You can attempt having a friend or private detective call them, posing as an employer calling for a reference. Sometimes, in speaking to a prospective employer, former employers will let their motivations slip.