Over on Dreams in the Lichhouse there is a discussion of time-spaced wandering monster encounters turning into massacres. In other words, wandering monsters (and wilderness wandering monsters in general) show up far enough apart that players can safely expend the maximum amount of resources to blow the encounter away and then rest back up to full strength.
There are a fair number of ways to address this - time constraints, limitations on resources (example: AD&D spell components), multi-part encounters (the monsters are scouts for a larger follow-on group, the dragons are a hunting pair and the other will come rushing, the bandits are camping and waiting for their buddies to come back from hunting, etc.), and so on.
Another is to introduce the idea of one encounter attracting other encounters. I've done this before and it's not any special idea of mine. But I realized I could use some specific rules for it for GURPS. These haven't been playtested yet, so consider them an idea not an endorsement or or a finished product.
Simple Method Whenever a monster is encountered (set or wandering), roll for wandering monsters at the the standard level for the area. Roll another die - 1-2, the monsters show up during the fight, 3-4 soon after (1d x 1d seconds), 5-6 well after (1d x 1d minutes). Add +1 for wilderness encounters.
In any case, either use your wandering encounter table or have a nearby monster come investigate (your choice). To decide how they react, roll a reaction roll normally for both sides, so the new monsters may either attack the PCs, the monsters, or both.
Repeat for each encounter and each group that shows up!
Detailed Method As above, but roll again at the next level down (if the area is 9 or less, roll at 6 or less). Apply the following modifiers:
-3 if the encounter is effectively silent, invisible, or otherwise difficult to detect.
-1 if the encounter is especially quiet or hard to see.
+1 for a noisy, visible, or spread-out fight.
+3 for an especially noisy, visible, or spread-out fight (or, optionally, +1 for each very nosiy/visible/large area combat event - flying creatures, explosive fireballs, screams, horn-blasts on the Horn of Valhalla, guys yelling "Bree Yark," etc.
+1 for a long fight (30+ seconds)
+2 for a very long fight (1+ minutes in GURPS)
+3 an extremely long fight (10+ minutes in GURPS)
Roll for arrive time and reactions as above.
For AD&D-ish games Add +1 to the "X in Y" roll for each increased chance of an encounter. So 1 in 12 becomes 2 in 12, 3 in 12, etc. Or you can reduce the die size for a big jump - 1 in 12 becomes 1 in 10, then 1 in 8, etc. down to 1 in 4. Roll a normal reaction for each side, using the standard encounter reaction tables. Additional monsters might attack either, both, or neither side.
Notes This seems pretty workable, and it might be fun. The simple method is pretty much implied, sans the rules for when they show up and reactions, by the standard wandering monster tables. The rest, well, extra detail can be fun for especially complex fights.
If you try this out please give me any feedback on how it worked out.