They are self inflicted injuries. As funny and down to earth as he seems, Collins is high maintenance and has admitted contemplating suicide in the past. He had three horrible divorces which cost him a lot of $$$ and stress. His back injury, which was caused by him falling over while drunk as he was depressed by his third divorce, has relegated him to sitting on a stool and singing.
Yep, I read Phil’s autobiography, I had no idea he was drinking so much.
And the thing is, all the other members of Genesis, including Peter and Steve, still all look great for their age.