can someone please tell me why do dogs sometimes eat their own poo ???

It can be for a variety of reasons - poor toilet training may have taught the dog to dispose of the evidence, there may be some lack in the diet (although this only applies to dogs who eat other animals feaces), the diet may be too rich - the dog detects some nutrional value and nature has geared them up not to turn down a valuable resource, the may have copied it from another dog  - often the mother dog, boredom.........  the list is endless!

If the dam has been left with the pups too long the pups see mum clearing up and join in.