There was that recent PR article that made the rounds here last week from some company allegedly developing an evacuated tube based system that was written in a way to imply they were working on Musk's idea. I would not at all be surprised if he's a little miffed about that and has decided to just get everything out there.

Given that they may not actually be working on what he specifically has in mind, there is a risk that his idea gets a bad connotation if someone implements something similar (but not identical) and does it poorly. There is a whole political dimension to this in terms of getting zoning, approvals, etc... and the last thing he needs is for it to be stillborn because a bunch of politicians hear it "was tried and failed".