Part of the essential life cycle.
Each chipmunk builds a burrow where it may live for up to several years. Chipmunks mostly stay away from each other's homes. Its tiny burrow is also a place to hide from predators. To make its burrow, the chipmunk digs a tunnel and fills the space with fluffy seeds, leaves and grasses. Underneath this bedding, it hides nuts and seeds for winter. That can help the nuts and seeds it does not eat grow into plants.
I would have been BMX crazy if these would have existed back then!