Why is this baseboard paint blistering and pulling away?
There are a variety of reasons that this could happen from humidity levels in the home to cheap paint to mixing a primer with an improper top coat.

However... what you are describing sounds like your windows are leaking.  I think that water is penetrating under your windows.  The crack is probably due to water sitting in the framing causing it to swell until it dries (if it does).  

I would pop that whole run of baseboard off and take some pictures for us.  The paint on the baseboards plus the baseboards cracking where it meets wall plus the other stuff going on makes me think you will find mold behind or the start of mold.  

From there we need to fix the root cause which might just be some caulking or an inherent issue with the window and you may need to redo a little drywall on the interior.  But I doubt this is just a paint issue.