Evolutionarily Stable Strategies

Now here's an interesting story. Reminds me of how China and Taiwan got to the point where China would shell two Taiwanese islands (Quemoy and Matsu) on, say, Mondays and Wednesdays, while Taiwan would shell China on Tuesdays and Thursdays.

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http://en.wikipedia.org/wiki/The_Evolution_of_Cooperation

The book included two chapters comparing Axelrod's findings to surprising findings in seemingly unrelated fields. In one of these, Axelrod examined spontaneous instances of cooperation during trench warfare in World War I. Troops of one side would shell the other side with mortars, but would often do so on a rigid schedule, and aim for a specific point in the other side's trenches, allowing the other side to minimize casualties. The other side would reciprocate in kind. The generals on both sides were satisfied that shelling was occurring and therefore the war was progressing satisfactorily, while the men in the trenches found a way to cooperatively protect each other.

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The question I have is, how did this arrangement occur, especially if there was no pre-arranged cooperation? Also, was this a strategy that saw repeated implementations across the front, ie did it happen just once, or did it happen in lots of places?

(Which reminds me of a similar question I once asked about high schools: why is it that the social structure of high school is so similar across different areas in the United States? Is Hollywood fostering the stereotypical view, or does the stereotypical view drive Hollywood?)