They 'don't like the cut of your jib'. They probably don't even need to give a reason.
I remember talking on this subject 15+ years ago with an old friend. As he said, if the IO flicks through your passport and finds ovestays almost on every other page (I had another old friend who was the overstay-king!), he might take a dim view. Also if you've submitted the passport to aThai embassy for a visa. And this was in the days before time spent in Thailand really became an issue.
I probably have 2 or 3 overstay stamps over a 20 year period.