The Curious Case of Graydon Hoare

I wonder, why Graydon Hoare, the author of Rust, stopped contributing into it and switched to Swift?

I burnt out; ran out of emotional energy to be effective in my role as technical lead for the project mid way through 2013, so I took a break, switched off the Rust team, took a year to work on less-time-sensitive projects inside Mozilla, eventually quit Mozilla and worked for a completely unrelated payment network [...] for another year and a half, then finally in early 2016 got a call from someone at Apple saying they were looking for some folks to help with Swift (in a non-leadership position, which I prefer).