This is what happens when jeans worn long instead of having the hem taken up before wearing them. The problem is this can take one time only for the back to fray away. The issue now is trying to create a jean hem. Because the back section is so frayed, the amount of hem allowance has been reduced.
Normally a jeans hem allowance is 3 cm (1 1/4 in). The hem is folded over twice. In this case, the alternative may be to overlock the edge after it has been cut and turn it up once. This way the jeans will be as long as possible.
The second alternative would be to put a false hem on the jeans. It will make the hem thicker and it will not fall as well but will give that much more in length.
To see how I alter jeans with the Jean Genie go to this blog post.