What Are the Possibilities?

I'm not a doctor, but I'm wondering if she might have had a mini-stroke and something in the brain went down. Like inhibitions. That happens.

the other way to find out what's going on is to ask HER. When she starts to get moody, just sit with her and say, "Mom, what's up? You've seemed upset lately." And then listen, and when she stops talking, pick up a thread from the last sentence and re-state it or use it in a question. The better you listen, the more she will share, and she probably knows what's going on.

If she is healthy, I wouldn't recommend interfering in her doctor app'ts. My mom is 85 and drives herself to the MD, and does all that stuff herself. I am 54, and I sure wouldn't expect my kids to show up in the examining room 10 years from now, just because I got frustrated with life and threw a few pots around.

If there are more symptoms than that, I would encourage HER to talk with a doctor, to see what's up, but the first step is to talk to her.

If 40 years of being a doormat is the cause, it means that she has been sad all along, and angry, and now it's escalating. There may or may not be medical factors involved, but it doesn't sound from your note like she went from happy and fine to angry and throwing things, which would be more indicative of a medical condition. Sometimes people just get more and more bold in their ways as they get older-crankier for people who were always sort of unhappy.
At an advanced age especially, it's hard for other people to help someone with how they feel. She would need to renovate her own soul and do some forgiving of herself and others and switch some priorities or do whatever is necessary to live happily in the present if she wants to. But you really can't make her.
All you can do is be genuinely nice to her. Not phony, but give her some true appreciation for things in her life. Find ways to spend time with her and lift her up. If you know of any interests of hers, maybe try to do one with her or buy a related gift... Push the boundary even further if you have a close relationship. Maybe help her take on something "new" by starting a new little family tradition of some sort...But definitely give her some "out of left field" compliments on the life she's given her son and his character etc. Ask her questions about herself.
If she's been a doormat, have people mistreated her? Step in and compliment her and diffuse rudeness from others whenever possible. Point out her strong points to her in front of others. Keep an eye on her and see what happens. If it gets worse, re-assess for medical signs etc.
Passive aggressive people sort of like to be wounded if it goes on too long, otherwise they eventually move away from the source of the attack even if they don't stand up for themselves. If this is the case, she may not want to feel better, so your kindnesses will be for nothing, but do your best, it's all you can do. Good luck!

S.,
I think behavior like that can be indicative of dementia, maybe even Alzheimer's. How is her memory? Is she acting obsessed with anything (the mail, cleaning, the weather forecast, etc.). Do you or your husband go to her regular dr. appts. with her? If so, you might want to mention the personality change to her MD.

Dear S.,
You said that your MIL is passive-aggressive, but throwing things & having tantrums is just aggressive ~ there's nothing passive about it. My mom went through a "phase" during menopause where her moods became more pronounced particularly when her hormones fluctuated. She tended to be more sensitive to my father's lack of sensitivity, and she was aware of it. However, she NEVER threw things. It sounds like someone needs to have a serious heart to heart with your MIL. If you have a close relationship with her, it could be you. However, if you've not had a close relationship in the past, now is NOT the time to approach her about the changes in her behavior, let that to your husband or his father (if he's still in the picture).
Lastly, if this behavior occurs near your children, they should immediately be removed. If questions are asked, you can try to politely explain that grandma's having a bad, etc. I have had to do with my one sister on several occasions. I have two kids & she doesn't have any. On a few occasions she's had a "screaming, hissy fit" in front of my kids while we were at my parents' house. I have always allowed one warning as privately as possible to both my parents (b/c it's their home) and to my sister (b/c she's the one behaving completley inappropriately). Last year on Christmas Eve, we had to go to my parents' house so my dad could help my husband with a car problem. This same sister "went off on me" in a screaming tirade b/c she didn't like the way I handled a situation with my son. We left immediately. I called my mom later to say that if she acted like that during our Christmas celebration the next day that we would do the same thing, regardless of whether we'd had lunch or opened presents. I refuse to expose my kids to that kind of behavior, as it is both verbally & emotionally abusive.
My apologies for being so "wordy". I will pray for you & your family as you deal with this tenuous situation.

Although I hope it's not the case here, loss of inihibitions (saying/doing inappropriate things) were the first signs of Alzheimers for several of my family members. You may want to get this checked out by her MD, if possible. I hope very much that this is NOT the case for your MIL.