All numeric vs alphanumeric PIN URLs

A small amount of my pins have super long mix of numbers and letters as their URL. Typical pin URLs are just numbers. Anybody seeing these alphanumeric URLs and know what's going on? I feel as if my pins are being tracked for some reason. And these URLs look clumsy. The rankings don't seem to be affected as at least one of my pins remain in the top search and still pulls quite a bit of traffic.