The biggest influence on the server's markets will be how players behave, especially its raiding schedule. Most of a server's players will be in its time zone or within a few hours of it; thus, assuming you are as well, a lot of the players will follow patterns similar to yours. A lot of a server's players will be people who are either working a day job or are at school during the day (a lot of the game's players are high-school students). While each server will have its fair share of shift workers, college students, people who work at home, and others that can log in at nearly any time of the day, most player activity will be concentrated around the evenings and weekends.
Furthermore, Blizzard sets daily and weekly cycles through daily quest and raid resets and, in the long term, with patch and expansion cycles. Combine the human factor with the resets imposed by Blizzard, and you get predictable day-to-day, week-to-week, and patch-to-patch market cycles.