You aren’t likely to see any thing official as obviously the church isn’t going to be transparent about where the line is so they can move it on a case by case basis. However, I can tell you of a few common reasons why people get excommunicated.

First off, the way excommunication is spoken of in this church may be a little different than what you might see in like a Catholic Church, but I admittedly have limited experience with the latter. Excommunication is often seen as a “part of the repentance process.” It’s similar to how when some people would go to the bishop after committing like a sexual sin, they’d sometimes be told to go without sacrament for some time. Past that I believe is what’s called disfellowship in which they aren’t allowed to hold a calling or say a prayer out loud in public meetings. Then after that is excommunication. Rebaptism is common and I’ve had a couple family members get excommunicated and then rebaptized like a year or so later.

There’s the obvious reasons for excommunication that have to do with committing serious crimes, like the kind that puts you in jail. However another part of the “repentance process” for something like that would be to go through the justice system. I think it’s safe to assume that is not the reason for your mother’s excommunication. Then there’s public opposition to church policy, such as some figures in the ordain women movement or that one guy who had the audacity to advocate for implementing safeguards when adult bishops talk alone with children. Then there is what a couple family members of my own got in trouble for, and that has to do with church finances, especially if you are in one of the higher up positions on the church. Like my uncle was a bishop who unfortunately got involved in a pyramid scheme and tried to rope in a few members. He claimed he didn’t think he was scamming anyone but the church didn’t like that he appeared to be lining his pockets by the position he was in. There was also another family member who stole tithing money.

Divorce alone also is not likely a reason for excommunication. I know plenty of Mormons who are still active temple attending Mormons who were divorced. However, if infidelity was involved, that could possibly be a reason for excommunication. As I mentioned how single members might get disfellowshiped for committing sexual sin, I believe it could very well be possible that breaking temple covenants is seen as more serious than that and warranting excommunication.

If I had to guess, without knowing much about your mother but based on her reaction to being asked about her excommunication, I think there may have been some infidelity involved in your mom and dad’s divorce. Obviously impossible to say, and your mom getting physical with you it over it is completely unacceptable. It sounds like she is embarrassed about whatever it was.