Let me repost Rot Grub's post from Paizo forums in full, because he's
completely on the money describing the issue:
"I am running 5 Paizo adventure paths at the moment (Age of Ashes, Agents of Edgewatch, and 2e conversions of Rise of the Runelords, Shattered Star, and Hell's Rebels). I enjoy Paizo APs for the wealth of story, locations, encounters and other material that I can tailor for my needs.
However, I see a prevalence of "monsters sitting in their room waiting to die" in most dungeons in APs. I like to play monsters with at least a modicum of intelligence, so when they hear that life-and-death struggle in the next room, they will consider joining in or at least check out what's happening.
This is more of a problem in 2nd Edition, where combining encounters makes for Extreme and often even more-difficult encounters when you combine them. And 2e's expectation that the party will take heal between encounters makes the idea that NPCs will stay in their room for 2 hours while their burglars/murderers sit down and "heal up" in the next room all the more unbelievable.
In Age of Ashes as we enter the final encounters of Chapter 2, this has been really prevalent. I chalked it up to growing pains with the new RPG. But as time has gone by, the AP designers seem committed to this pattern of moderate to severe encounters with intelligent enemies being clustered together. (Spoiler for the new Beginner Box: they encounter kobolds early on who are written as being unaware of the party, even though the party had just had a fight about 80 feet away, in an otherwise-silent dungeon no less.)
It's not like the designers are unaware that monsters can respond dynamically to the PCs' actions: there are a number of places, particularly in Age of Ashes volume 2, where the text describes how enemies will respond to a fight breaking out in other areas. (Still, even running as written it can quickly get out of control, as there are about 11(!) encounters, most of them ready to reinforce some of the others, all in a single open-air area. With 2e's tight encounter math this can quickly get deadly even with smart play.) )
It seems as if the designers of 2nd Edition have found a winning formula for making individual fights tense and exciting, but the AP designers are designing dungeons like they used to in 1st Edition, where you could combine encounters without killing the party. You could gather 12 Goblins in 1e, but that Fireball or that Black Tentacles could handle it. Not so in 2e.
So what I do, is when preparing for an area in 2e, I anticipate when encounters will combine and lower the difficulty of individual fights with the expectation that some of them will combine. It keeps the dungeon dynamic, and it rewards the party for finding ways to isolate groups of enemies. 2e at least has the advantage of giving us the tools so we can predict how hard things will get.
Still, I think it would be better if the designers didn't assume that every monster sat in their room waiting to die. The overarching stories in Paizo's APs can be strong; this seems like a story weakness."