They did not necessarily come to avenge the death of their king like they did in the book, as the name suggests they were mercenaries hired by Sauron's chief orc lieutenant Azog and the latter's son Bolg. It is also possible that with the king's death, they were leaderless and regrouped under the Dark Lord's banner.
I forgot to add that not all of them where sent along the troops of Gundabad as some were seen alongside the Guldur Orcs, Guldur War Trolls, Guldur Olog-hai, Great Guldur War Trolls, Wargs, Bats and Ogres from Moria, Mount Gram and Goblin-Town that made up the Guldur forces.SwtorPlayer (talk) 15:48, March 8, 2018 (UTC)