That people are calling it when you enter a hold and everyone attacks you? I'm in Markarth and the Jarl, his steward, housecarl and the three thalmor immiedatly turned hostile towards me and are attacking me. The guards are not. I'm not sure what is going on, I am only coming back with the Jarl's father's shield that I got from inside Blind Cliff with the friendly hagraven...

The glitch usually happens after you escape cidna mine. If you dont kill the king in rags they all escape and kill the guards. He says he will clear your name but it doesnt regester in game so that guards keep comming after you.

I havn't done that quest and the guards aren't hostile just the nobles in understone keep.

do you have a bounty? or are you a vampire? maybe you upset the silverblood family? theres quite a few glitches/reasons that it can happen its hard to nail it down to one thing

I have done this twice and never killed the king in rags and got no problem, however I have always been passive in the fight in the city.

No bounty
Not vampire
Never harmed anyone save for the guy that tride to kill Margaret
Completed Jarl's first quest then went to retreive his shield
The cave that had the shield was home to a non-hostile hagraven that wanted me to free her and help her kill another hagraven.
Came back everyone was hostile.