Since the draft has recently occurred, Andrew Brandt over at SI.com wrote an article that examines some of the implications of rookie contracts now coming in en masse. The new CBA has expedited the process of signings since it has removed the incentives of the previous CBA for agents to wait until July to sign.
Part of the reason why players and agents would wait is to avoid players in a similar slot in the draft "jumping their slot." Such a thing would hurt an agent's ability to recruit draftees in the future. But, now, with the signings much more regularized, the players don't need to pay an agent his maximum signing fee (3%), and Ereck Flowers is even negotiating his contract with the Giants by himself.