The earliest exoduses were socialist political communities banned from reddit (particularly /r/chapotraphouse who formed Hexbear, and /r/GenZedong who landed in Lemmygrad). Then, the most recent exodus is related to censorship related to Luigi Mangione, a US political issue, coinciding with a strong sudden re-emergence of global anti-American [government] sentiment due to diplomatic catastrophes with a range of former allies.

In fact, the founders cite reddit's corporate nature and its pro-US-imperialist, racist stances as their motivation to create Lemmy: https://join-lemmy.org/docs/users/07-history-of-lemmy.html

So I'm not surprised at all that the default feed is covered in political topics, it's always been a strong topic here and it's just gotten stronger. But there's probably enough activity now that one can filter it out and still have enough action to keep it fun.