He has a lot of medical issues really. Like Faceless says, he has a back problem, mainly his spine, that prevents him from sitting down for long periods of time. I believe the maximum amount of time he can sit comfortably is 15-20 minutes, which is a recent improvement if you can believe it, so he wouldn't be able to sit on a plane for a couple of hours without pain flaring up. A lot of other issues occur off of his spine problems as well, like his neuropathy, but he also has other shit like arthritis. His hiatus was indeed because of his back, but apparently, it was because something went wrong. He gets spinal injections so I'm guessing something went wrong there, but he didn't go into much detail beyond being stuck in a hospital bed and said hospital having terrible internet.