The answer is obvious. We do the same thing whenever we step in mud or wet sand. The dinosaur steps in mud and leaves behind a footprint. However, in the dinosaur's case, that footprint became a fossil... well actually each footprint often becomes two fossil footprints. Both fossils come from the one footprint. That's what you see in the above photo. Both of those fossil dinosaur tracks came from the same footprint. Notice how they are mirror images.
Here's what happens: the dinosaur steps in the mud and leaves a footprint. A new layer of sediment is deposited, filling in and covering the footprint. The sediment turns to stone. Then, when the rock is split open, the bottom half is the depressed footprint made directly by the dinosaur's foot. The top is a natural cast, a raised impression of the footprint. Both are considered original fossil footprints.
Impressed / Raised Tracks
The above sketch shows the two tracks, with the upper raised track lifted off and set to the side. There you have it. Two fossil dinosaur tracks from a single footprint.
Sounds simple, but real-life is more complicated. For instance, in my example the rock split at the sediment layer the dinosaur's foot touched. This is called a true print. But the rock does not always split where we'd like it to split. First, there is no way to know where we'd like the rock to split. We cannot see inside the rock to know if, or where there is a dinosaur track. Secondarily, the rock will only split at a weak point, such as where there is mica or a biofilm.
The Rock Splits Where There Is Mica
The tracks in our museum are in a sedimentary rock known as shale. In this case the shale will split where there is mica. The above photo shows a close-up of the edge of a slab of shale and you can see a crack. If we wanted to see what was inside this piece of shale we'd tap a wedge into the crack and gently split the rock. This rock will not split anywhere else. Only where it has been weakened by mica.
Notice in Sketch A (above) that the layers of sediment beneath the footprint are bent. This is because when the the dinosaur stepped on the sediment, the pressure of its foot was transmitted into the underlying layers of sediment. These underlying sediment layers were bent into less distinct copies of the footprint.
Dinosaur Tracks - Underprints
If the mica is below the layer the dinosaur's foot touched, the rock will split at that lower location. This reveals a less distinct impression of the footprint called an underprint.
Sketch B (above) shows how an underprint is revealed. Underprints have less detail than a true print. While a true print may clearly show foot pads and skin impressions, underprints do not have much detail. At times they may retain the impressions of all three toes and the claws, but in other cases the underprint may be barely recognizable as a dinosaur footprint.