Lots and lots of little things. I've said before that in my case, it was death by a thousand cuts and then suddenly you get to the last one, which might only be tiny, but it really is a case of the straw breaking the camels' back.
It doesn't have to be a big thing OP, something that you can easily justify, sometimes it's simply an accumulation of many, many small things.
In hindsight I wish that I'd picked up my exH every single time, but when it's happening you go into forgiving / accommodating / understanding mode - I did anyway.
So to give you an example, on our very 1st Christmas dinner, which I'd spent hours planning and cooking, his sister rang just as we started eating and he disappeared for 20 minutes (they were/are very close), leaving me to eat dinner on my own, despite me gesticulating that he should ring back when we'd finished eating.
I should have thrown a mega strop and put his dinner in the bin. And then explained why I was so cross. I didn't, and that was where it started to go wrong - it took 10 years to finally come to an end.
It doesn't matter what it is tbh, if it matters to you, it's important.