The NYT Science section has a nice article on some recent thinking about the Anasazi. About their mysterious migrations, the writer says:
"Scientists once thought the answer lay in impersonal factors like the onset of a great drought or a little ice age. But as evidence accumulates, those explanations have come to seem too pat — and slavishly deterministic."
This touches on the problem of granularity, which I struggled with while studying history in grad school. In essence, if you look closely at a historical process or event, everything seems very chaotic and unpredictable, with unnumerable chances for things to have gone differently. As you step farther and farther back, certain things take on an air of inevitability and those small decisions seem to matter less and less.
The less information you have - which often means, the longer ago we are studying - the more those big forces (like climate change) seem to dominate. As you fill in details (personality conflicts, random events), contingency takes center stage. Rather like gravity and quantum mechanics. Somewhere those two worlds intersect, but I never had a solid grasp on balancing them against each other.