Ok, I went over a bit of code in more detail. I analysed the function at address 0x11003D00 in e09.dll. This method is called from InitProc, and seems to setup the triggers and victory/failure conditions for the level.
Slight correction over my earlier post. There are 4 rects, each of which leads to an ambush. The rects are initially disabled. They are enabled by a CountTrigger. If you have greater than or equal to 4 cargo trucks carrying wreckage, the ambush rects are enabled.
The 4 rects, and the corresponding trigger callbacks are:
//Rect(x, y, width, height)
Rect(110, 96, 8, 9) => Ambush1
Rect(147, 78, 3, 8) => Ambush2
Rect(41, 94, 15, 19) => Ambush3
Rect(112, 16, 3, 4) => Ambush4
Both players have a reinforce VehicleCountTrigger. It is a one-shot trigger.
If Player 0 has fewer than 8 units, call "ReinforceE"
If player 1 has fewer than 8 units, call "ReinforceP"
In addition to those triggers, there is a CountTrigger for player 0:
If player 0 has greater than or equal to 4 cargo trucks with wreckage, call "ReinforceE".
That should imply player 0 can get two sets of reinforcements, once when low on units, and once after picking up all 4 pieces of wreckage.
There is a "ToggleP" trigger. It is called periodically by a repeating TimeTrigger. The frequency depends on difficulty setting:
Easy = 5000 ticks
Medium = 3000 ticks
Hard = 1200 ticks
The "ToggleP" callback has some code involving FightGroups. It uses DoGuardRect, and SetRect. There seems to be a static Rect variable, which is initialized elsewhere with the values (81, 115, 101, 126). The method makes use of the player difficulty setting and random numbers. It references "Reinforce1", "Reinforce2", and "Reinforce2Easy". I've not looked at this method in much detail.
The main trigger setup function also sets up disaster triggers for various disaster types, at various intervals. A few looked interesting.
If player 0 has 3 or more cargo trucks with wreckage, call "Quake3".
Tornadoes vary by difficulty setting:
Easy, Medium = 12000 ticks to 18000 ticks
Hard = 5000 ticks to 6000 ticks
Storm: 5000 ticks to 11000 ticks
Quake1: 8400 ticks
Quake2: 24000 ticks