I have actually seen this happen a couple times; there are 2 ways I have seen it done..
1 is the way that a lot of errors come about; someone disconnects while game is loading, which screws up variables.
What most likely happened, is there was a fast/extreme creep spawn; and whoever was using that spawn sucked, and was running in and out of it, over... and... over... and... over... And they were summoning more creeps; which in turned summoned more creeps; while other creeps chased the noob out of their creep spawn, over and over; thus changing the units in that region to a low number (the rest ran out) so some spawn, and the others run back... And assuming more units that cast summons were spawned; they keep growing exponentially. Most ive ever gotten up to while trying to abuse this was somewhere around 30.
But yeah; this cant happen in 4.0 because i use dual regions to check where creeps are; and they would have to go VERY far to leave my "security region"