Why do my 2 puppies gang up on our older dog?

Me and my mom had 2 dogs. 2 boys, but tragically one passed away and we still have our other. We got 2 new puppies to help deal with the grief and they are both Pomeranian puppies only 1 day apart. One is a boy and one is a girl. They have not been spayed/neutered because they are too young. So, whenever i put our older dogs leash on both the puppies start barking,jumping on and snapping at our older dog and we have no idea why! It happens every time! I feel bad for our older dog because with 2 puppies its hard to keep both away! Do you have any idea why??