The May 12 date was one that was likely circled on the calendars within the Miami Dolphins organization. Under NFL rules, that date means unrestricted free agents (those who became free agents via expiring contracts, not by being released) no longer count against a team when the league determines compensatory draft picks for the next season. The Dolphins, due to the losses of players like Olivier Vernon, Lamar Miller, and Rishard Matthews, stand in position to pick up at least a couple of the compensatory picks for the 2017 NFL Draft.
According to multiple reports throughout the offseason, the Dolphins have been very aware of their compensatory pick projections. They have flirted with a few veteran players on the free agency market, but did not sign them, likely in part because of the compensatory draft pick considerations.
May 12 has now arrived, which means every team in the league can start signing any of the players that still remain in free agency, without risking possible draft picks for next year.
There is a lot of salary cap space still available around the NFL, and with the Miami Dolphins in particular. According to the NFL Players Association, the Dolphins have the eighth most salary cap space in the league, trailing teams that have ridiculous amounts of salary cap space like the Jacksonville Jaguars, who have over $54 million in cap space; the San Francisco 49ers, with $49.5 million in space; and the Cleveland Browns, with $43.2 million. They have the most cap space in the AFC East, where the Buffalo Bills have $14.2 million, the New England Patriots have $9.8 million, and the New York Jets have $4.0 million.
Miami currently has about $18.5 million in cap space to spend. Four rookies still need contracts, which will eat about $2.5 million of that space, giving the team around $16 million in space to spend this year. Assuming the Dolphins want to keep around $5 million for in-season operating costs and to sign players as injury replacements, that would still give Miami about $11 million in space to add some veteran free agents.