This is actually quite common and it called Coprophagia. Some dogs do this because they can smell the undigested protein in the poop. Some people suggest putting unseasoned meat tenderizer in their food because it has an ingredient that breaks protein down so dogs will not find the poop nutritious. Be sure to use the unseasoned variety because seasoned has too much salt. Here's a link with more information.
Stopping Dogs From Eating Poop There is also a product sold at pet stores that stops this.