Specific reasons, I would not pretend to know. I can guess on a few. Probably Willis saw the writing on the wall at Vandy and saw that he was not going to be one of their top tier pitchers or get much scholarship money from them. Maybe he wanted to come back home and go to a northeast school not far from home. If that is the case, UConn has to be in consideration as being the top baseball school in the northeast, and there is certainly an appeal in that. Plus the UConn coaching staff has certainly developed a reputation for getting players ready for pro ball and the MLB draft, especially when it comes to having pitchers who have potential who they develop and end up getting drafted.
I imagine that once Justin Willis gets back from that eventful Florida vacation, he will probably be joining his UConn teammate Zach Bushling and meeting up with him in Keene to play with the Swamp Bats. Willis has played summer ball with the Swamp Bats before. I'm not sure, but this could be his third summer season with them.