According to my seamstress sister:
If the zipper comes apart while its zipped up, the zipper is just plain worn out, and there's no solution short of a new one.
If its just happening when you unzip it and get to the bottom, its usually because the fabric supporting the zipper has become so old that its lost its "body", and allows the thingee at the end to flop out of the way when the zipper mechanism hits it as you unzip. About the only solution is unzip it VERY carefully, and hold the bottom thingie in one hand while unzipping with the other, to make sure you don't unzip too far.
Would like to placate the OT Patrol by tying this to tractors, but I'm coming up empty- no, wait- If your canvas cab or Weatherguard or Heat Houser has any zippers, you might want to take heed! There.