The risk with any trades and free agents was once explained on ESPN radio. I don't recall which former player said it but it went like this.
When players are drafted, the initial welcoming to the NFL for the young guys is with their new team. That organization, coaches and players, quickly feels like family. The family feeling is very strong, since they are being taken under the wing of the organization. There is a certain comfort zone that is attained.
When players get traded or sign as free agents that feeling of family is gone and now the player is in the league as more of an individual. Some players struggle with the move as if they were pulled from their family and are slow to adjust to new surroundings. For some the fun is gone and they don't play at their previous level.
My brother was a high school standout and when went to college to play he didn't like it because it wasn't fun for him without his friends. He quit the team and chose a tech school near home. His experience leads me to believe that the same feelings may be true for many other individuals including Pros.