Guyton is yet another one of those guys who flashed strong ability and then faded. I always wonder why players like that can't muster consistency.
Think about the discipline it takes to dedicate your life to staying in shape, and in fact improving your strength and conditioning.
There are very few players in the NFL that don't have to be 100% dedicated to their workout regimen, nutritional habits, and avoidance of substances or activities that are bad for them.
Add in the fact that most grew up dirt poor, and even the UDFAs make more money than 99% of American wage earners, and its easy to see how they can lose their edge.
I think a guy like Guyton is easy to envision as someone who worked his way onto a roster by being focussed and dedicated and passing by other players who gave less effort. By the same token, when he takes a step backward in effort and commitment, others pass him by, and that fine line of NFL caliber vs out of the league is crossed.
Its easy to see in any field how employees become comfortable, develop bad habits and coast. While they can turn it back on, and NFL player has guys waiting to take his job that are just as talented, and its awfully hard to ever get back the lost improvement of slacking in workouts and development.