After sleeping on it, here on my thoughts on Fast. There are a few possibilities as to what happened:
1) The Rangers offered Fast the same contract as he would ultimately sign with Carolina, but Fast wanted more from the Rangers
2) The Ranger offered a shorter deal with more money and Fast opted for more years.
3) Fast saw his spot with the Rangers are uncertain as prospects/younger players start moving into the lineup and he wasn't sure what it meant for him — then decided he was better off leaving for a contender.
4) Fast was insulted by the Rangers assuming they would get the chance to match any offer.
5) The Rangers were insulted that Fast, who reportedly wanted to stay, was shopping himself around then using other offers to negotiate.
5) Jeff Gorton and John Davidson are really bad at their jobs and their diabolical secret motive is to destroy this team from the inside.
Personally, I think some combination of the first 4 is the most likely case scenario.