Who is it? Is it other roving bands of orcs? I don't really understand.

Orsinium has been sacked twice (or maybe three times) in its history, each time by the combined forces of the Ra'ga of Hammerfell and the Bretons of High Rock. The next question is likly why? well Bretons and Redguards are racist and despise Orc kind.

At least one sacking was do to Orc raiders from Orsinium that was cutting off travelers and trade carravans coming into High Rock, and the Ra'ga hate elves and it could be that they mistook the Orcs for the Goblin Frander Hunding defeated after the Ra'ga came to Hammerfell from Yokuda. The other two times it was sacked was likely rooted in past prejudice.

thanks, i just never really understood

Also just to add.

Orsinium used to be in the mountains of High Rock up until the last time it was sacked. The Orsimer survivors went east into Skyrim then south into the mountains between Skyrim and Hammerfell which where they have built the new Orsinium.
The Orcs have recently appealed to the Empire to recognise the new location as a non-provinent state of Orsinium. A city that doesn't fall under the authority of another province and has self control

If the Orcs were raiding trade caravans going into High Rock, why does the Breton attack and expulsion of the Orcs constitute 'racism'?

I don't have any particular love for the Bretons, but in this case it sounds like the Orcs were asking for trouble.

Because the Bretons and Ra'ga at the time considered them to be nothing but monsters, no better than goblins or other undesirable yet possibly dangerous creatures. The fact that they continue to hold a grudge against the Orc's of Orsinium past the origional sack is proof enough that it goes a little deeper than them have acosted traiders a few hundred years ago.

Of course we all know they were best of friends in the second era interregnum. Thanks TESO for clearing that up for us. Ah, well.

Meh