Unless you were sick and lost that much, it's scientifically and physiologically impossible to gain 50lbs of muscle in a year....but, hey, you look good anyway!
Yeah, many factors when into that. Drastic change in quality food/calories, stopped smoking, muscle memory. Plus, that's not lean mass, so it would be 35lbs I'm guessing. Now, I'm happy gaining 5lbs. a year of quality weight. If you have a regular routine with a mediocre diet, then diet solid for a year, you could probably put on 20lbs, I'm guessing.