Why do the Pirates have a Cubs batting donut?

Neil Walker is a Pittsburgh guy through and through. He was born and raised in the Steel City before being drafted by his favorite team and still plays for the Pirates today. But something looked off about the Bucs second baseman's on-deck circle warmup Saturday: his batting donut, bearing the logo of a completely separate NL Central franchise.

Any ideas on how the incongruous weight made it to the Pirates? Is a member of their clubhouse simply "a thief" as Giants color analyst Mike Krukow suggested? Or is something more superstitious going on?