Rust drops at the same time. I don't use Arch, so I can only guess it was something related to the way Arch was distributing the packages. Maybe they fell behind or something?
I use arch and I don't have the rust package installed. In it's stead, I have the rustup package installed. As the rust project recommends the use rustup to manage multiple possible installed versions of rust, my guess is that most arch users only install rustup.
I'm not sure if the values for rust on this graph account for the rustup package.