As announced in a joint statement, the "NFL Management Council and NFL Players Association have determined that the Seattle Seahawks violated the Collective Bargaining Agreement's offseason workout rules on the prohibition of live contact during one of the club's organized team activity days ("OTA days"). As a result, the Seahawks will forfeit two of their scheduled OTA practices (June 6 and 7) as well as an additional offseason workout day on Friday, June 8. Seahawks' players are not permitted to be at the facility on those days, but will be paid for the sessions. The club cannot reschedule the cancelled days."
It's not clear when the Seahawks violated the live contact rules but as Andrew Brandt surmises, they might have gotten a little too overzealous. With new practice rules outlined in the CBA, contact has been curtailed quite a bit during the offseason and apparently the Seahawks pushed their luck a little bit on this one. All practices are videotaped for further review by league and union officials.
With practices now cancelled tomorrow and Thursday, it's unclear when the Brian Banks tryout that was scheduled for the 7th will occur.
UPDATE: Pete Carroll responds to the announcement here.