New mother trying to bury one of her puppies

Question
My black lab mix gave birth (w/ full yellow lab) 10 days ago to 11 puppies (2 were still birth).  We found her with her first puppy under her outside kennel.  We moved her inside with the first puppy.  Two days ago she took her first born out under the outside kennel.  We've rescued it twice now.  Tonight she took the same puppy and tried to bury it alive in another part of the yard.  She has not done this with any of the other puppies.  She still cares for and nurses this puppy.  Any ideas on this behavior?

Answer
hello it is possible since she had her first puppy outside that she now wants to keep this puppy outside. THe best thing to do is when she goes outside close the door so she cant drag puppies away from the house then open the door to let her back in, Bar off doggy doors also if you have one.