I meet Orcs on the road all the time. I've even had them show up in some Inns. They always open with a dialogue calling me a "Milk Drinker" and ends up with them lying in a pool of their own blood afterwards.
I was invited to one of the Orc strongholds after a quest but never went. Maybe if you don't visit the stronghold that you've been invited to they take it as an insult and want to beat you up?
No, there's Redguards and Nords that do the same. Same dialog and all.
Same end result too. One day they'll figure out there's less painful ways to commit suicide.