I like the way Scott Boras describes an opt-out clause:

A three-year deal between the Yankees and reliever Rafael Soriano, signed in January 2011, gave Soriano the right to opt out after the first or second year. Soriano had just earned 45 saves for Tampa Bay but did not have a long history as a closer. When an injury to Mariano Rivera thrust him back into that role in 2012, Soriano pitched well, opted out, and got a richer deal with Washington. Boras called that structure the “drive-through” contract.

“That’s where you have a three-year guarantee, but you can drive through the window and you can either pick up the Happy Meal or you have the choice of getting back on the freeway and trying to find an eight-course dinner — the long-term deal,” he said. “And surprisingly, the Soriano contract has the most value to the majority of the players in the game, and no one talked about it.”

Teams became very good over the years at controlling player salaries through long-term contracts, escpecially to young players. The player side is taking some of that control back. I also suspect the players are speculating that a lot more money is coming into the game as local TV deals keep growing. This is a way of them preventing being locked out of an inflating salary structure.