He's been part of some very pivotal oss projects/books/talks so people like to follow him to see where he's going and what he may be working on. I'm sure there are a huge number of engineers he's worked with/mentored who would also be interested.

I actually follow fitz all over the place but didn't realize he was part of Subversion so I'm seeing this from the other end and get to trace back to his previous stuff.