The cold hard facts of this trade are becoming clear to everyone now. The Pats could have kept Collins on the team for the rest of the season, even if they benched him, let him walk, and got a 3rd Rd compensatory 2018 pick. That seems like the reasonable thing to do even if you had soured on his on-field execution. There will be injuries and it might be nice to bring in Collins off the bench. Or use him as a 3rd down replacement, or mop up time at the end of games. I don't see how he could be disrupting the team so much that you would basically give him away. But of course I probably don't know the whole story.
The only possible explanation is that the Pats feel they can trade or pick up a replacement for his spot on the roster that is more valuable. It may not be a linebacker but getting rid of Collins does open up a roster spot. If they fill that spot by taking someone off the practice squad, i will be even more perplexed. That would mean that they felt the practice squad player on the bench is better than Collins on the bench. Wow, hard to see how that is possible. Now if they pick up a veteran, someone like a Chris Long but maybe OL or some other position, then this could make sense, a little. It still means that they think Collins is the 53rd best player on the team.