I don't have a great answer for this. I mean, it's natural for there to be ebbs and flows and balance. This specific timing definitely isn't preordained.
The way it seems to work is that Riot tries to reshape problem points in their game every winter. That reshaping typically is looking at three areas of the game:
- Support isn't fun / can't buy items / ward burden
- Early junglers too influential / farming jungle not sufficiently rewarding / only a few viable junglers
- Top lane is stupid / melee-ranged interactions
So far, the only one of these they've gotten to feel pretty good is #1, and that took two seasons to get it right. They're coming after #2 harder than ever in preseason 5, and who knows, maybe that'll work. #3 is borderline unsolvable, and that's the one that always gets us into trouble on this topic.
In any case, what tends to happen is that Riot overshoots the mark on whatever fixes they're trying to make to the game. In season 3, they wanted to move from a resistance-based tanking to a health-based tanking. That was a success, but suddenly the first guy to buy a Giant's Belt was crushing top lane and the League of Warmog's was on. Since bruisers tend to be the most broken thing about the game at any given time, it's only natural that we're trying to fix them every winter, so given this overshooting tendency in their attempts to fix things, it's not too surprising that bruisers come out overtuned every time their itemization gets tweaked.
That's my best guess for why the timing is as it is.