The over justification effect happens when external motivators such as money or prizes decrease someones intrinsic motivation(motivation from within) to do a task. When someone is offered money, prizes, or any other external reward for doing something that was once done based purely for the joy of it, their motivation shifts from intrinsic to extrinsic motivation. When the external rewards are no longer offered, interest decreases significantly and the intrinsic motivation does not "come back."
In more basic terms, if you pay someone to do something, they are now motivated by money/ prizes and not by themselves. When you take away their pay they will not have the same interest in pursuing the activity and often times don't have the same intrinsic motivation.
No one wants to admit that their love for skiing may have decreased, but does it happen? Sponsored skiers, has this happened to you? Other industry people, does this include you?