I've noticed that several projects on the front page today (and over the past few days) are migrating away from GitHub.

Is there any recent event or broader trend that explains this shift?

Zig’s announcement[0] might provide some insight

[0] https://ziglang.org/news/migrating-from-github-to-codeberg/

Ongoing availability issues, Microsoft's shoehorning of AI, GitHub's focus on migrating to Azure infrastructure rather than adding features and fixing shortcomings. If I had to guess.