Cats lifting their bums when you scratch them is a common behavior that often makes us chuckle. This action, known as "bum scooting" or "butt scooting", is a way for cats to express their pleasure and contentment.
When you scratch a cat in a spot they enjoy, like their back or behind their ears, they release endorphins, which make them feel good. This feeling of pleasure can also trigger a reflex to rub their scent glands, located on their flanks and near their tail, on the surface they are enjoying.
This scent marking behavior is a way for cats to communicate their presence and ownership of their territory. By rubbing their scent glands, they leave behind a unique scent that identifies them.
While this behavior might seem strange to us, it's a natural and instinctive part of a cat's communication and social behavior. It's a sign that your cat is happy and comfortable in your presence.