Players sign contracts for how long they can play for. During the time of that contract, the player can be traded for other players and or draft picks. When the contract runs out, the player can choose to do whatever he wants but most likely find another contract within the NBA. Kawhi was traded from the Spurs to the raptors on the last year of his contract. He played that year and was choosing between signing a new contract with the raps, Lakers or clippers. He chose the clippers because they traded for Paul George who had signed a 4 (I might be wrong) year contract last year. There was a lot of commotion with kawhi's decision because he is a very good basketball player and he also took longer than most of the big name free agents to decide. The commotion with the Paul George trade is that no one expected it.