The optimal composition is 2 sololaners, (usually, one bruiser, and one AP carry), 2 duo-laners( ad carry and support) and one jungler.
The bruiser is usually top, the APC mid, and the dual - lane is bottom.
Switching lanes is a metagame, it was something very unusual a few months ago, and now it is quite common.
Metagame changes may come from several things:
- Discovery: People start to realize that a character is awesome when played correctly. And in a month, this character is perma pick or ban. Thi happened countless of times: LeeSin was almost never played at first, neither was Skarner or nunu top. Until we realized their true potential.
- New items: when an item is modified, the characters using it are either buffed, or nerfed. Take black cleaver and his buff several months ago. It made it very strong, so strong that you could play a team with 5 bruiser, each, with 5 blacks cleaver. In the same way, the new black cleaver made Rammus almost unplayable, while Chogath had became one of the most resillient tank.
- New strategy: people once discovered split push. That was something that was almost never used right, until people realized they could use the TP spell of a character to split push, then go back to your team to fight 4v5.
- Counter strategy: first there was a "normal" composition, which was beaten by AOE composition (Galio, MF, FiddleStick, Amumu, etc), which was then beaten by hypercarry team (team aiming to protect ONE hyper adcarry, such as Kogmaw or Vayne), which was beaten by assassin team (Akali, talon, kassadin) which was beaten by strong bruiser teams, etc...
- Other seemingly meaningless updates: When Ezreal got his ultimate skin, it has such a great success that everybody started to play him, and Ez was back to being a monster ADC.
These are just a few changes that changed the metagame.
EDIT (thanks vogel): What I wanted to point out is that metagame changes often appear after a game update. Although changes are often initiated by players (mostly top players, to be honnest), but some of them are wanted by Riot (ie: nerfing oracle, to make the game more dynamic and aggressive, giving AP jungler items to make them viable in jungle).