The Shelton and Dorsett moves are hardly surprising, considering their histories in New England--Dorsett made a marginal impact last year in sub duty, and Shelton was just acquired this offseason. Without a proven track record for either player in Foxboro, spending $7.1 million on each of them for one more year wouldn't have been a financially prudent decision. This way, the pair will have this season in New England to impress the coaches and negotiate a longer-term deal--otherwise they'll become free agents at the end of the year.
The Brown decision came more out of left field. The big DT out of Texas has started 37 games for New England in the past three seasons, and has quickly established himself as an integral part of the defense. In all likelihood, this is just a strategy to help get a new deal done, but losing Brown in free agency would be a much bigger blow than Shelton or Dorsett.