The BSR component used by FG doesn't pertain only to SB's, but rather to baserunning efficiency in general. You only asked about stolen bases, and i gave you the correct answer. Also, Trout had a 3.3 overall FG DWAR while Cabrera had a -14.8 mark, so i don't know where you got that -.8.
I don't know if you're using BB-Ref's formula, for which the BSR component is a bit more SB-reliant, but still, the main reasons Trout had such a big WAR advantage are clearly both defense and baserunning.