The Chargers traded first-round picks with the Dolphins (in addition to sending a 2nd and 4th Miami's way), and selected RB Ryan Mathews with the 12th overall pick. While this may have been a reach on San Diego's part, they obviously felt the need to secure a #1 RB with the decline and then departure of LaDainian Tomlinson.
Scout Andy Rioux listed San Diego as the best fantasy situation for a rookie RB last week:
The LaDanian Tomlinson era is over in the city that stays classy, and while Darren Sproles will be back, the speedy Sproles isn’t capable of carrying a full load. He certainly isn’t a consistent short yardage touchdown threat. San Diego needs a workhorse who can diversify the offense the way LT used to, and fantasy owners need to strongly consider [Ryan Mathews] as he would potentially play an integral role on a great offense.