I believe, as seen with the High Elf/Wood Elf conflict, that things are being taken into OOC. I want that to stop.
Let's take some things into mind here, shall we?
Orcs are creatures who thrive in and heartily seek out warfare. While you may call their slave-raiding of your city baiting, or the presence of a slavemaster similar, I would attribute that to the fact baiting, in character, is in their culture. Orcs seek combat, and are not adverse to goading others into it.
Although you might argue they do not have proper casus belli for this skirmish, I think it is not only logical in RP but plausible for such a response to occur, even stemming from the smallest of actions done against them such as a few orcs being killed.