I think mine originally stem from misunderstandings. I can have a lot of misunderstandings through the course of a week, but it doesn't guarantee I will have a meltdown for each time, and it doesn't mean I won't blow at the end of the week. I think all in all for me, it has a lot to do with things being so complicated, with each person being so different and thinking I have found people to talk to then having a big misunderstanding with them, etc. Usually I blow when someone starts a discussion but refuses to clarify something for me because they think it serves no purpose... or when after a week of misunderstandings, I misinterpret something to be moreso of a personal attack to something I cannot change. The rest of every day things just seem to be build up to it. I probably have a lot of small ones I am fairly unaware of, as I tend to go through a lot of times where I just need to be alone for a little bit altogether or I start blocking things out.
Just as an NT has something similar to a meltdown (except it usually comes out as anger and an unwillingness to listen) when they are explaining something that is very emotional for them and I cannot relate-by my questions they are somewhat forced to think about things they would rather not... mine comes out differently, but it is essentially the same thing I think. Theirs is due to my lack of "caring" as they say, mine is usually due to their lack of caring or their blissful ignorance to seeing what is going on factually, or to clarify what they are saying to me so that I can understand while they are deeming me "inconsiderate".
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Sorry for the long post...
I'm my own guinea pig.