It happens every year. One team has unreasonably high expectations for their team, signs a couple of free agents, and when they fail to meet their goals, they become sellers at the trade deadline.
Need an example? How about last year's Seattle Mariners. In theory, Cliff Lee would have been great if they had an offense. However, their lack of success forced them to trade Lee to the Rangers, a team better suited for Lee.
On the other side, there are always buyers at the trade deadline. Maybe a team doesn't expect to be in playoff contention in July, however now that they are, they need one more player to piece together the puzzle and put them over the edge.
A good example is the Milwaukee Brewers of 2008. It was their acquisition of CC Sabathia that helped them reach the playoffs through the wild card.
Another common way for players to switch teams midseason is by under performing. By August, if a team is not content with the way one of their players is playing, they have to option to put him on waivers.
An example of this is the Alex Rios scenario of 2009. The Blue Jays, unhappy with Rios, placed Rios on waivers, where the White Sox picked him up. His .199 batting average in 2009 with the White Sox probably made the Blue Jays happy they let him go.
There are many other reasons that a player is moved during the season— these are just the main ones. In this article, I will speculate as to which players I believe will be moved in 2011.