In the case of contract negotiations, I'm always wondering if there may be some desire from one or even both parties in terms of waiting to develop a larger sample size, but that's just a guess. For example, if the Pats are offering 10m dollars a season, Collins could theoretically shortchange himself by taking that offer right now. With another strong season, he may be able to bump it up another 15-20 percent in AAV, with even more guaranteed money.
I also think that the team is somewhat player friendly in terms of allowing guys to hit free agency in an attempt to develop proper market value. As we know, that can backfire, just as well. Sometimes they choose to match another offer, while other times they do not. As you mentioned, an ideal situation would be to try and lock him up now, based on his first 3 years of production and progression. For all we know, they may have been trying to do that for awhile now, with Collins' agent playing hardball.