Monsters don't have to stay in their rooms.

 An enduring and old complaint about D&D and similar RPGs is "what are the monsters doing in those rooms?". Well, they don't have to stay there.

Many published dungeons seem to treat the monsters in any given room and the entire room itself as if it is frozen in amber waiting for the PCs to turn up. The used of pre-written description text sure doesn't help dispel this notion (I recall one adventure with 6 or 7 paragraphs describing a room and in the last sentence it mentions the 1st thing PCs would notice... a Beholder). Many critics, DMs and publishers seem to forget that while the published dungeon may be fixed and as is on the page the monsters don't have to stay in their rooms.

 For reasons of record keeping and to keep the pressure on the random encounter with wandering monsters is an old tool to make the dungeon feel more menacing and alive.  You know what also does this even better... the monsters don't have to stay in their rooms.

A dungeon comes alive if the residents react to the presence of PCs storming through the dungeon. The setting gains verisimilitude and the excitement cranks up form 7 to 11 if the monsters react to the presence of the PCs by not staying in their rooms.

Are there different factions in the dungeon? You know what explains that better to the players than anything else: seeing a member of one faction double-cross or take advantage of another faction while the PCS are interacting with them. 

Do monsters have their own motivations? Show it by having the kobolds in room 12 sneaking in and carrying away the loot in room 11 while PCS are fighting the gnoll guards that came running from room 10.

Are the PCS making a LOT of noise... you know by kicking in a door and killing everything on the other side while maybe blasting off a spell or three? They should hear all the nearby monsters threatening them, nailing the door to their room shut, raising the alarm, or rushing to attack the PCs.

Verisimilitude is more important than trope. Monsters don't have to stay in their rooms.

I've had players freak out and get into the game even more when the actions of their PCs triggered 4 or 5 (or more) rooms worth of monsters to show up during a fight. 

If you want more than simple murder-hobo dungeon crawling you have to give the players a reason to consider stealth, scouting, research, and diplomacy. Monsters reacting to changes in their environment before the PCS otherwise get to the monsters as things happen goes a long way toward this. If the players do want a big massive multi-room battle sprawling down hundreds of feet of hallways that's' great...monsters don't have to stay in their rooms.

This happened in a session I dm'd maybe 18 years ago:   

The party of 10 adventurers was strung out down a corridor. One player wanted to open a certain door at one end of the group... another wanted to pen a different door. There was three or four other doors and passages in view. One door was opened... oh no a group of orcs! The fight begins... the gnolls behind the other door at the other end open the door to see what is going on. A pcs panic that a third door near them has something dangerous behind it too and starts to spike it shut. The orcs are dispatched and the fight with the gnolls is going poorly for the gnolls, one of the party members at the end of the encounter near the orc room is swallowed by a giant snake attracted to the commotion... oh no ... more gnolls turn up forcing the party to react, they deiced to force the door one of them spiked shut open... oh no ... a wraith is in there. The party choosing to escape the wraith maneuvers away from it and the giant snake.... the wraith slays the snake and the PC that was swallowed and not yet dead. The ghouls down the way lay and ambush hearing all of this going down... all of that happening in maybe a dozen or so rounds of game time and a wild hour or so at the table. All much more exciting and dangerous than having the monsters stay put and wait for the PCs to march from room to room. A few of the gnoll wander back to their lair and run away with some of the tribes loot. Monsters don't have to stay in their rooms.

Have the monsters react to changes and noise as it happens and as they are capable. Remeber the monsters motivations and habits the scenario may show their location and maybe even spell out their motivations but that can change as the environment around them changes. The monsters have movement rates... get them moving. Monsters don't have to stay in their rooms.