I know this sounds like a crappy possibility but I had a 2000 Mustang GT a couple years ago. One day I was coming to a stop and when I pushed the clutch down something popped. When I tried to take off it ground but finally went into gear. I got back home by shifting without the clutch and when I pushed down the clutch to stop when I got home, but it wouldn't stop, I had to pull it out of gear. I tore into it that night and the pressure plate had broke and snapped in-two and dug into the clutch disc. The clutch fork was also broke where the two little fingers held it on the retaining ball, I am not sure which caused which but you might look to see if your pressure plate cracked and broke. Mine looked like it was release but there were 2 points that where still in contact with the clutch disc.
When everything was said and done, I had to have new clutch kit, new clutch fork, resurfaced flywheel, and new clutch cable (because the crappy factory adjuster broke) and this car only had 50,000 miles on it , but somehow it just broke.
Maybe you can check and see if the pressure plate is released all the way around, or if it looks like it is "bending" somewhere and still touching.