Per the San Diego Union Tribune:
Rivers was due to earn $12 million in base salary this season. The restructure adds no new money to the quarterback’s contract that runs through 2015, simply transferring $5 million of that $12 million into signing bonus. The bonus becomes prorated over the life of the three remaining years, hence the approximate $3.3 in new space.
The move was reportedly made so the Chargers could sign a player, likely to re-sign linebacker Donald Butler to a contract extension.
Butler and the Chargers had been working on an extension the past few weeks but there was never any indication that a deal was close, however, things may have changed as Butler likely would prefer to avoid contractual discussions lingering into the season.
It’s also possible the Chargers have their eye on a free agent, though, Butler’s extension seems to be the more likely scenario.