Anyone else had that?

It's happened to me twice now, in Riften and in Markarth. I have no bounty in either place (or at all - I play a goodie-type) and suddenly a bunch of guards run up and start attacking me. Not all the city guards, just some of them. They didn't give me the chance to yield (I was unarmed anyway) or try to arrest me, no dialogue.

The time it happened in Riften I reloaded an earlier save and that sorted it. This time, I got annoyed and killed the guard that attacked me in the temple of Dibella (no, I wasn't stealing the statue) and after that they're all ok with me again. AI?? Hah...

Did you happen to kill any Vigilants of Stendarr, Imperial guards with Stormcloak prisoner, Thalmor with prisoner or wandering Stormcloaks on the road? Or cast spells on them?

Probably. However, does that explain why most of a hold's guards have no issue with me, but the odd few do? And why would the guards in Markarth care - or know - about anyone I picked fights with on a road in Whiterun?

It's worth noting too that in 350 hrs of gameplay I never started a fight; though I usually finish them

That would explain it. It happens to me as well. It's like getting a bounty for killing someone but then they are killed and the bounty is removed. The guards in the hold will still attack you. Just hit the wait button and waiting for a few in-game days should take care of the problem if it occurs again.