Mounting (when one dog places his paws and chest on the hindquarters of another dog) is a normal and very common canine behaviour. In most cases, it is NOT a sexual behaviour unless the dog is thrusting.
In young puppies, it is a play behaviour. In older dogs, it is a method of communication, the dog who is mounting the other dog is generally displaying a heirachy.
Quite often owners who have brought another dog into the family get upset when they see the younger dog mounting or biting the rear legs of the resident dog. It is best not to correct the behaviour,
it is very normal. Sometimes this is when the status of the young dog is changing in the pack and the younger dog becomes the top dog.
Trying to understand this behaviour it might be worth thinking how children as they grow into adults take sometimes take care and make decisions for their parents as they get old. The human pack also changes!
My daughter always bosses me around!