Your Dog Thinks They're On the Pad. Their Back End Has No Idea.
When a dog's front paws land on something, their brain says they're there. What's happening behind those front paws doesn't get noticed the same way.
Puppies are still learning where their body ends. Senior dogs lose that sense as they get older. Either way, a dog can step onto the pad with their front feet and have no clue their back half is hanging off the side.
That's not them being difficult. That's where their body ended up.