Oh this =) I've seen this heaps of times. Judging from the map you are playing, it's a line wars of some sort that you can spawn an unlimited amount of creeps on the screen.
The problem is intermittent?
I think you'll find that it is related to the amount of units on the screen at one time. That happens when Warcraft hits it limit and can no longer keep track of everything on the screen - mine can do it too (with line wars/tower wars as well). If you want to try to reduce it, go to Warcraft's options and lower Model Detail. It will draw the units with less polygons and less detail textures, hopefully allowing more units on the screen without this happening.