The director in L4D was had a very simple logic with inputs like number of alive special infecteds and number of zombies to control spawns. It's like seeing a health kit when your health is low.
If it thought players had had it easy for a while it might throw a larger waves of monsters and specifically deny spawning health for a few minutes.
There's a good presentation somewhere about how it would try to balance the rising action with periods of "stress" and "relief" over the course of a level.
Have to start somewhere, though.