I think he was getting arbitration expensive and ready for a multi-year extension so, yeah, salary.
Scherzer & Verlander were coming up (lost out on Scherzer), as well as Miggy. And they ended up trading Porcello also, the next year I think, to afford their stars & scrubs model when they were already pushing the luxury tax so... yeah, salary.
In order to spend money on the BP and save money for extending JV & Miggy and (potentially) Scherzer...
Yeah, they were pro-active in trading Fister. As someone else noted, he may have already been losing his FB a bit, so there's that too. But salary considerations were definitely a part of it.