This one is pretty odd... It looks like due to an accounting error, the Eagles accidentally overpaid Westbrook $3 million.
As part of the five-year extension Westbrook signed in 2005, he was guaranteed $9 million. He received $6 million shortly after signing the deal and was supposed to receive the other $3 million late last year. The Eagles recently realized they had paid him another $6 million instead.
Had the Eagles not come forward, the league could have suspected that they were paying Westbrook under the table and penalized them by counting the overpayment against this year's salary cap. Because they have yet to be repaid, the Eagles were required by league and the NFL Players Association to file a grievance.