Free agency includes departures and arrivals that start on first day of the NFL new year, but the 2015 version on Tuesday was highlighted by multiple unforeseen trades that commanded attention across the country.

ESPN.com's Kevin Seifert wrote about the uniqueness of the headline-capturing swaps.

Seifert called Tuesday's activity "the most unusual and intense scene imaginable," citing St. Louis sending quarterback Sam Bradford to Philadelphia for QB Nick Foles, New Orleans sending tight end Jimmy Graham to Seattle for center Max Unger and Detroit trading to acquire Haloti Ngata from Baltimore to absorb some of the loss anticipated by the likely departure of Ndamukong Suh, whose deal hasn't become official at the time of this post.

Minnesota's trade of reserve QB Matt Cassel to Buffalo also was finalized Tuesday, and more trades across the league could ensue.

Seifert offered a couple of reasons for the increased flurry of trading:

NFL general managers used to limit player trades in part because of salary-cap implications. When a player is traded, the prorated portion of his cap figure — mostly his signing bonus accelerates into the current league year. In some cases, that acceleration created an intolerable cap hit for a player no longer on the roster. Over the past two years, however, the salary cap has risen by $20 million. Surplus cap space is easier to find. Plus, teams have found other ways to limit proration in long deals. Long story short, trades are much less prohibitive now from a salary-cap perspective. Many of us want to attribute Tuesday's deals to a new generation of swashbuckling general managers, which might be true in some cases, but the trend has a more tangible base than that.

Here's a secondary reason that trades have become more popular: When the opening bell rang Tuesday, the NFL announced 453 players on its official list of free agents. Based on records kept by ESPN Stats & Information, that's the lowest total since 2009. (There were 477 last year.) Teams are no less interested in adding players this year, so they have pursued other avenues amid the shrinking pool.

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