I've been meaning to write this out somewhere. I may have, but I can't put a finger on it now.
Way back when we started in Felltower, a group of hobgoblins controlled the access to the dungeons. The PCs encountered them a few times, and killed them.
They also fought the orcs that held another portion of the dungeon. Both sides opened up with hostile actions.
In the end, they'd wiped out the hobgoblins and made a pact with the orcs, paying a toll to enter the dungeon in return for peace with the orcs and the orcs leaving them alone, plus occasionally providing them with a guide or help. The orcs took advantage by occupying more of the dungeon, including the surface, and fortifying the damaged castle above and making it an active, actual fortress again.
Predictably, though, the PCs immediately started to look for another way into the dungeon, a toll-free one . . . and made sure to kill any orcs they found to cover their tracks. They justified this by rationalizing that their agreement with the orcs only applied if they paid the toll, and that otherwise killing orcs was fine.
They often continued to pay a toll and work with the orcs. They even had a joint encounter with the Lord of Spite, which was going pretty well before the Lord of Spite backed off and left them to their casualties. There was also some grumbling that the orcs hadn't held up their end of the bargain by not fighting hard enough against the Lord of Spite.
The orcs, not being total fools - and the PCs doing a pretty sloppy job of concealing their actions - they understood the PCs were bypassing the tolls and were the ones who'd killed some orcs in the dungeons.
They raised the toll and got pretty hostile.
The PCs refused to pay the toll and got violent back.
The PCs pushed deeper in the the orc-held areas, and killed any orcs they found. "Let's go kill orcs" became a way to spend a session, prep the ground for future delves, and a backup plan to get some loot at the end of a session.
The orcs tried a few ambushes of their own - sealing off the PCs with the Lord of Spite, launching multi-pronged attacks with goblin shock troops, monsters, spellcasters, elite orc Slayers, and so on. The PCs fought back and generally shattered the orc's attempts.
The orcs tried to just block off sections of the dungeon with rubble, preventing the PCs from reaching them without delays to dig through. The PCs dug through, pretty routinely, and killed the orcs they found.
To be "fair," the PCs did offer to the orcs to take on whatever task has drawn the orcs to Felltower and complete it for them, in return for a reward. Then, the orcs could leave. The orcs refused, for reasons the PCs don't know. Of course, the delvers took this as proof that they couldn't work with the orcs. The orcs clearly have a goal they need to accomplish, but which they either don't think the delvers can - or don't want them to - complete on their behalf. So they won't go.
So to start with, the PCs really wanted the orcs to just stop harrassing them and let them delve deeper without any interference.
Once the orcs stopped harassing them, the PCs wanted to make sure they couldn't do so again.
They attacked them until the orcs simply did not mount any offensive action against the PCs, and abandoned the upper works and gave up shooting arrows at intruders to the dungeon.
Once they forced the orcs onto the total defensive, they tried to wipe them out. That failed, and their negotiations to get the orcs to agree to never fight them and, essentially, stay out of Felltower, failed.
Since the PC's big attack on the orcs, the orcs haven't mounted a serious challenge to them. Perhaps any challenge to them. They haven't harrassed them, attacked them, or otherwise did anything offensive against them. They did reinforce their passive defenses very strongly - going from rubble blockades with barricades to magically-solidified stone blockages.
It just so happens that the best places to block off the PCs from the orcs also block off the PCs from easy access to the "touch only once" altar and the lenses. Not that the PCs have anything remotely like a plan with the lenses, but they want to be able to go and fiddle with them. The orcs, by blocking the PCs off from them, have been deemed to have inflicted a great and unforgivable insult to the PCs.
The PCs have made plans to break those blockages, and spent a recent trip on finding a way to the orcs around the blockades so they can attack them. To what end?
Will the PCs ever be satisfied vis-a-vis the orcs?
I think no, not until the orcs have completed disintingrated. Not been reduced as a threat, not as a hassle, but completely gone. Every explanation has always been "+1" - we want X. They get X. Well, we really need X+1. They get X+1, Well, yeah, but X+2 is actually what we need here. And so on.
The PCs really seem to want total, unfettered access to everywhere in the dungeon, and the orcs - if they are there - to fulfill the role of keeping other delver-hostile beings from occupying any part of the dungeon. Perhaps that plus the orcs providing a source of treasure on top of it - either by handing it over or giving the PCs "quests" - a common ask - to earn money from the orcs. Naturally, if they found anything they perceived as more valuable than the offered quest reward, they'd expect to be able to keep that, instead of, or in addition to, the quest reward. Pretty harsh, but I don't think my players could claim any of this isn't true.
It's tough being a monster in the face of greedy delvers. The demands of delvers are not unlike that of an empire - they just want a little more, and then come back again for a little more on top of it.
With this in mind, I expect that the orcs vs. the PCs will be ongoing for as long as we keep playing Felltower. It's not something I expected from day one, although I did make it clear that the orcs have a fundamental conncection to Felltower. It's just amusing to watch the delvers explain why, really, their demands are reasonable and the orcs are being unreasonable as they change their demands every time they achieve them.