Simply put, because of gravity.
The sensor measures impacts based on the amount of acceleration applied to your valuable. When your object come into contact with another object, a shock wave is sent through it triggering an impact reading. The intensity of the shock wave increases with the intensity impact. The intensity of impact is represented in g's.
So what is G-force? G-force stands for either the force of gravity on a particular body or the force of acceleration anywhere. It is measured in g's, where 1 g is equal to the force of gravity at the Earth's surface. The g-force on an object is its acceleration relative to free-fall.