this may not be relevant since you mentioned your disk is still showing lots of free space, but i have had a similar situation of a disk inexplicably running out of space. in my case, it turned out adobe after effects was creating a cache file that was growing out of control (like 70gb) as an animation was rendering. seemed to be a one time glitch in the particular file i was working from, and the cache file deleted itself when i quit the render process.
anyway, perhaps do a spotlight search for files larger than several gb the next time you get the error; your particular torrent software might be creating a similar caching error?