Are we speaking about more than 100-150 units?
If yes this is a "bug" in wc3's pathfinding. Only a certain amount of units per player can be moved at a time, so for many units you will have these malfunctions.
I guess the problem is on the enemy side of the 10 hero siege map, as we have a lot of spawned creeps there typically.
You can split the enemy side to several players to increase the amount of units being able to move properly. If you split the creeps to three players for example, you tripple the amount of creeps you can have.