Despite what some may think, those in Zhentil Keep haven't forgotten about their orc troops in Phent. In public discussion in Zhentil Keep, the leaders of the Zhentilar, the military branch of Zhentil Keep, have confidently asserted that the orcs in Thesk are completely loyal to Zhentil Keep. They maintain that the orcs are just biding their time and building up trust among the citizens, until the appointed time when the word is given.
In private, these same leaders are gravely concerned. The leaders didn't get to positions of command by being idiots, and they know that the orcs are treated well and accepted in Thesk, which is a rarity for them with the humans and humanoids of Faerûn in general. The leaders know that many of the orcs would be reluctant to destroy the source of this acceptance.
But what if the orcs' chief god, Gruumsh, told them to?
The Zhentilar turned to the Black Network and presented the problem. The Zhents knew what to do. The Zhents have dispatched a powerful cleric, a master of persuasion and deception, to pose as an orc prophet of Gruumsh and whip the orcs into a destructive frenzy. In addition, the Zhentilar have staged raids against human caravans by what look like orc warriors so that they can start antiorc sentiment among the population of Thesk.