mtr, what would you say drives those cycles?
I have no concrete reasons, but a number of hypotheses.
1)
Publicity
Jibaku's Rising from the Storms Team is a classic example of this. Giratina-O usage has skyrocketed while only the most conservative players employ Giratina on their stall teams any more. Stallish play in general is at a high point, mainly because the RMT was an inspiration to many.
2)
Players
Ubers doesn't have as many players as other tiers, so every individual player is important in his own right. If stall-loving players such as Uchiha, myself, and others play a lot on the ladder, stall play will rise. If we're sucking in Physics class and need to drop Pokemon for a bit, stall play will fall.
3)
Inertia
Once the cycles are in place, they're hard to stop. I go back to what I said earlier, with a few edits.
If stall becomes popular, as it is now, you can bet that things like ScarfKia and ScarfChomp will fall in usage because Scarfers aren't good against stall. Because these guys are falling in usage and stall teams are rising in usage, things like MixPalkia and SpecsOgre or even MixOgre will rise in popularity. Then stall declines, and offensive play rises. Then players decide to once again use tools to beat offense like the aforementioned ScarfChomp. Then those tools become popular and stall begins to rise...
Naturally, things are more complicated than this, but this is a distilled version.
There are many more reasons, like HGSS and Plat move tutors, the discovery of things like Jumpluff, but these are the reasons that the cycles occur.