HiCustomer

This is called fecal incontinence. Sometimes if a dog has had trauma to the rectal area such as continued anal gland issues, fecal incontinence will develop as a result of a loss of muscle tone in the sphincter. This can also be caused by an injury to the back or a problem with an invertebral disc. An intervertebral disc that has slipped or ruptured up into the spinal canal causes inflammation of the spinal cord, which in severe cases causes paralyses of the rear legs. You can read about this here:
http://www.petplace.com/dogs/intervertebral-disc-disease-thoracolumbar-area-in-dogs/page1.aspx
http://www.petplace.com/dogs/intervertebral-disc-disease-cervical-area/page1.aspx

It might also be a perineal hernia which you can read about here.
http://www.petplace.com/dogs/perineal-hernia/page1.aspx

Large dog breeds sometimes suffer from a condition called degenerative myelopathy which can also cause problems with the rear legs and paralyses.   You can read about this here:
http://www.petplace.com/dogs/degenerative-myelopathy/page1.aspx

You may want to have your dog checked over by your vet as back issues are best treated early for the highest chance of successful treatment.

A few things that can help with this is adding fiber to the diet to firm up the feces. A few spoonfuls of canned pumpkin (not pie filling) added to the food works well for this purpose. Also there is one drug frequently prescribed which is phenylpropanolamine which can help tighten the sphincter muscle. You can read about this here:
http://www.petplace.com/drug-library/phenylpropanolamine/page1.aspx
http://www.wedgewoodpharmacy.com/monographs/phenylpropanolamine.asp

I hope this information is helpful to you