Because the closure itself contains a reference either to the struct or to its fields. That's the second path, and it can be invalidated by the first path either a) freeing the struct or b) changing its "shape" in memory (resizing a `Vec`, changing enum variants, etc.)

You can alternatively move the struct or its fields into the closure, but then you lose the first path. I know you're not doing that because if you were you wouldn't be having lifetime problems (and you wouldn't be able to share the state across event handlers, so it's not a great solution anyway).