Why? You do work on the can by pushing it. This work gives
the can mechanical kinetic energy, so it rolls across the floor.
As the can rolls, the rubber band winds up, storing more and
more potential energy. This energy is called elastic potential
energy, which is a form of mechanical potential energy.
When the can stops, the rubber band starts to unwind, and
the elastic potential energy stored in it is changed to mechanical
kinetic energy, causing the can to roll backward. The can
continues to roll after the rubber band is unwound due to
inertia (the tendency of an object in motion to continue to
move forward), causing the rubber band to wind up again.
This winding and unwinding of the rubber band can continue
several times until all the mechanical kinetic energy is transferred
into other types of energy, mostly heat from friction.
Then the can stops.