Here goes my explanation.
These guys are lined up capitalists on the left and communists on the right. Yeah, I would have lined up the other way too but I think the hill is biased against capitalists and besides the capitalists were there first. Anyway, as soon as there more than a couple of communists get on-board they start falling over and the first thing you know you're witnessing the old domino effect.
Pretty good, huh?
Casey