I was just reading that too and there is a lot of detail to that story that is missing. He probably didn't comply with some rule of staying in the union (paying union dues, etc) and the union gave Sam Raimi a hard time about it. If anyone has a link to an article with Sam Raimi telling his version of it, I would like to read it. It may not be the most fascinating story either, just another production problem on a movie set. It reminds me of the problem Ridley Scott had with his American film crew after an article was published during production on Blade Runner. He was asked if he liked British film crews better than American film crews and it became a "thing" on the set and some people quit, some people made t-shirts about it. It has nothing to do with the movie but it's kind of an interesting production story.
As stika9 says, it was the Screen Actors Guild although years ago, there used to be The Screen Extras Guild but I think that has since been absorbed by SAG. There was also AFTRA which covered actors working in Television and Radio. As I understand it; SAG and AFTRA finally merged in 2012. The unions have all sorts of rules the productions need to comply with in terms of how much union actors are paid, OT, etc. So it sounds like they insisted he be paid the limit for a non-union actor until he resolved whatever issue there was.

The most common actors union should be the Screen Actors Guild, I think.

The Actors union I suppose. I guess actors have unions as well to help them out.
Jessica Alba ist die Schnste in Hollywood Land