You "might" be a half inch ahead of most people as you said you have a union, but still.
Yes to what BipolarRN said . She really nailed it.. The truth is that employers can do whatever they want and use any trumped up reason.. why? because they hold the power, the gold key and a office full of Lawyers that love holding things like this up for months into years.
Being fired is terrible , I have been there. Maybe there is a reason this has happened , maybe that job was keeping your ability to find stability just out of reach?
I'm sorry this happened