I remember people saying that randomly in major cities and outside, thalmor agents can track you down and attempt to arrest/kill you. How do you make that happen? I m no talking about the 3 thalmors and 1 prisoner scene, but rather a group of thalmors after your head.

btw i completed the main quest and dawnguard.

kill the ones escorting the prisoner. kill the ones you find walking on the road. kill any you come across. Siding with the Stormcloaks I'm sure would also do the trick...

Anything specific though. not sure. I kill every one on sight so... yeah they come after me. Elenwen is dead though in all my saves so... any now are following old orders.

Helps if you do the quest "Missing in Action." Talk to Fralia Grey-Mane in Whiterun to start the quest.

You get it by existing.

Seriously, there's no real trigger to get the encounter. They'll find you, sooner or later.

Just talk to them. Things usually go down hill fast after that. After enough times, they get the idea that you are not going to blindly follow their lead. For some reason, that is what really makes them mad enough to send hit squads after you. Which you can also do with the quests above.

Even then, I would get a few for a while, then go long periods without getting any.

Be anything but one of them.