The 2010 NFL season came one year before the expiration of the CBA and by rule it was designed an uncapped year, which meant there was no salary cap. The NFL, reportedly, instructed teams not to use that season as a salary dump year -- loading contracts with big payouts that year to keep salary cap numbers low when the salary cap returned in later years.
At least two of those teams apparently violated the rule. ESPN's Adam Schefter reports: "NFL is taking away millions of dollars of salary-cap space from Cowboys and Redskins for how they front-loaded deals during uncapped year." As Pro Football Talk points out, one such example could be Miles Austin, who received a base salary of $17 million in 2010. The idea is that the Cowboys paid him significantly more in 2010 when there was no salary cap, which means his salary cap number in future years would be lower.
As of Sunday, the Redskins had $31.1 million in cap space while the Cowboys had just $4.7 million in space. It's unclear exactly how much will be taken away.
It's hard to see the NFL's argument on this absent a clear explanation of the rule. If they wanted to avoid teams using the uncapped year to their advantage, then they should have come to a labor agreement with the players in time in order to avoid the uncapped year.