After Magneto blanketed the Earth with an electromagnetic wave, it short-circuited the world's technology. Believing Magneto had gone too far, the X-Men headed to his orbital base, Avalon, to put him down once and for all. When Wolverine launched himself at the X-Men's nemesis, Mags used his magnetic powers to rip the adamantium out of Wolverine's body, shredding him to pieces.
Because of his healing factor, Wolverine managed to survive. Unfortunately, his healing factor had to work on overdrive, reverting Logan to a feral state. As he developed more and more animalistic traits, Logan was terrified a day would come when he would lose his humanity and be unable to remember his old life or recognise himself and his loved ones. He was terrified he wouldn't be able to control his base instincts around the X-Men and so, quit the team.
His healing factor was overloaded so much by Magneto's attack, it shut down, leaving Wolverine more vulnerable than ever. For a time, he lived in the forest like a wild animal, killing and eating prey. He did eventually regain his adamantium skeleton and powers as well as his humanity but it was a long road to recovery.