Because they feel the ground is moving.
In animal consciousness, just as in Einstein’s theory of relativity, there is no such thing as an absolute frame of reference; in other words, something is absolutely at rest while something else is in absolute motion. We now know thanks to Einstein that there is no ether permeating all of space as an immovable backstop against which motion takes place. Everything is in motion and so saying what-is-moving-relative-to-what, is a judgment call. The classic example of this being two ships slipping their anchor in the harbor and then currents cause them to collide. Which ship ran into the other would depend on which vessel one is on. So while we may consider time, space and mass to be fixed values in our experience of reality, these are actually relative to one’s frame of reference and are in fact malleable according to deeper influences. Time and space is dynamic, not static, and while this defies and confounds our human reason, the animal mind is not organized in such a way where it must contemplate such paradoxes.
Since animal consciousness and emotion is predicated on the laws of physics rather than a human, mental construct of reality, this means that when a dog is attracted to something and that object of attraction moves, it feels to the dog just as if its physical center-of-gravity is moving within its own body, – just as if it is moving itself, even though it may be standing perfectly still. It’s exactly like a process of magnetic induction wherein it doesn’t matter whether one moves a magnet toward and around a coil of wire, or whether a coil of wire is moved toward and around a magnet; either way an electrical current is induced in the wire. As far as the wire is concerned, the energizing effects are identical.
Therefore, a dog has no idea that it is moving relative to something motionless, or that something is moving relative to it. It feels the exact same internal movement within its body either way. This is why a dog in a moving car might strike out at something going past. The dog has no idea that it is moving relative to something that might be standing perfectly still, rather, the dog perceives that something flashing past at 30, 40, 50 mph etc. is indeed running like prey. So when a dog encounters a slippery floor for the first time, it has no idea that it is moving relative to a stationary floor. The dog doesn’t understand that because its claws are tightly clenched, it is failing to secure a purchase and so is in effect running in place. Instead it perceives the situation as if the floor itself is moving. And the faster the dog tries to run to stable ground, the faster the ground seems to move, which can be as frightening to a dog as it would be for us to be standing on ground that’s heaving and shaking due to an earthquake.
Eventually of course, most dogs get over the problem, but not because they understand there was an error in their perception, as for example a young child might do after their first experience on an escalator, or Einstein did when he contemplated the nature of light, mass and time. Rather, the physical memory of “flow” eventually will paper over this “gap” of slippery-floor-as disconnect-in-consciousness, so that the dog is able to connect the feeling of terra firma from both sides of the slippery floor. Revealingly, there is a transitional phase of acclimation, rather than an all of a sudden “AHA” moment of realization. This is because the dog’s emotional battery and its physical memory of flow as synonymous with firm footing, is gradually filling the gap in consciousness that a slippery floor causes in the sense of being grounded. This kind of learning is exactly analogous to how we ourselves learn to walk across a patch of ice. We know that if we can just maintain a constant rate of movement without any displacement from a center line, this steady pace will smooth out the temporary glitch in footing. In fact, we quickly learn in a counter-intuitive manner, that if we gradually and constantly accelerate our motion in crossing over the patch, most of our energy will be directed forward and hence the side-to-side swing of our hips will be neutralized, making us less likely to slip. This is an emotional calculus predicated on physics and the laws of motion, and this awareness arises from our animal mind and is exactly how dogs learn to negotiate the floor. They steadily accelerate as they learn to focus on the feeling of flow from their physical memory bank, and this they come to feel is what prevents the rug from being pulled out from under their paws.