Lemmy was released as an open-source fediverse alternative to Reddit.
Just over a year after launch, r/ChapoTrapHouse, moved across after being banned from Reddit. This is likely what you’re referring to. It had well over 100,000 active users on Reddit, so represented a sudden sizable influx of users.
I’d wager the biggest influx of people by far, though, occurred when Spez upset a majority of mods and many users by banning third party apps.
People looked for an alternative, and Lemmy was it.
But why are so many people who lean left politically? Because the Venn Diagram for “people who like the idea of a decentralised platform that supports everybody and is free from the machinations of millionaires”, and “people who would like society that supports everybody and is free from the machinations of millionaires” is nearly a circle.