To be clear, if SHE wanted to move out to "work on your marriage", it really makes no sense - she can't work on it if she isn't there..
GENERALLY when they want to move out, it means she wants to bang someone (or more than one) else. She may have already had her partner picked out and she is blaming the marriage stuff on YOU so that she can justify the affair.
Check your cell phone bill to see if she has any calls to/from a number you don't recoginize and texts to a number that seems way too high. This is just a first step to see if she is having an affair.