Your description of your wife's behaviour reminds me a bit of my mother.
She is diagnosed manic depressive and takes medication. Usually she is a very nice person but at times of stress or worry she begins to change in behaviour. First, as you said of your wife, she will begin to become withdrawn and want to be on her own, then little snide and negative comments start to keep cropping up. It will then escalate to full verbal insults and put downs over nothing at all, such as not closing the kitchen door or turning a light off. The comments and can become very nasty and personal, and she begin to basically start an argument even though no one is arguing back with her. You can't seem to win, because if you argue back she goes ballistic, but if you ignore her, that REALLY winds her up because she hates being ignored.
She has been violent in the past during such episodes, however not towards people but towards objects and things.
Like your wife, when it is all over she claims not to remember what happened, nothing happened. There was no argument and no insults dished out. However i suspect she does remember, because for days afterwards she will start to buy gifts and other things for no reason at all. And so, the cycle continues over the years. It has gotten better in recent years but it still happens every now and then.
I wish you all the best with your situation.