There are players who just aren't good teammates. These guys usually have great natural talent and get to the pros because of that talent without putting the hard work in that most other guys have to in order to develop their own natural talent.
Being pampered at the high school and college levels and then getting showered with money upon reaching the pros can create monsters of some players, who feel entitled to do just about anything they want because, hey, I'm the boss around here.
These players let their egos run wild and turn into clubhouse cancers, their actions so destructive they end up affecting on-field performance and sometimes killing a team's chance to win a championship or to just make it to the playoffs.
Here's a list of the top 20 NFL clubhouse cancers.