This is such a misnomer when it comes to metagames. It's not a "herd" mentality per se. Higher level players figure out effective strategies because they have superior game knowledge to be able to fully test out the effectiveness of currently "off meta" choices. The success rate of those "off meta" experimentations trickles down to lower skiled players who are good enough to emulate existing strategies but are not skilled enough to experiment effectively. This becomes a "metagame trend." The reason why this isn't a "herd mentality" is that it's not a collective conscious deciding as a group that one strategy is the most effective. A small group has shown it's effective and players are forced into a play strong meta choice or beat strong meta choice with another choice scenario. The problem is most the community is not skilled enough to experiment with off meta choices so they're somewhat "forced" to pick strong meta choices or they ARE skilled but due to their game knowledge and other factors see no need to experiment. However there will always be a number of high level players who will try to beat the meta, and if their efforts prove effective the meta shifts. It's a cyclical thing and a meta that shifts gradually usually means the game is in a relatively healthy state. When a meta stagnates that's when there needs to be intervention of some sort from Riot.

First off people need to get off the notion the meta can be "right" or "wrong." The meta is fluid by nature, no one is ever picking the "right" meta, they're choosing the most "effective" meta. This distinction is important because numerous factors can can change whether a strategy is effective or not. A hypothetical scenario for this is let's say Ezreal is at the worst he's ever been, he's like a super low tier ADC which has been nerf hammered to oblivion. Except for some reason in his kit makes him effective against Graves. Graves is such a strong pick that his pick rate becomes insane. Naturally you'd pick Ez even though he's by nature a "bad" pick because the match up is favorable. Suddenly all these Ezreals start popping up because some new tech gets learned, other ADC's start to get picked, and then maybe everything shifts back to Graves. The point is these things are just randomly decreed, effort and time are put in by some players and metagames will always go from being solid, to loose and back again, it's the nature of the game, it's not because the population 'decided it to be.'