Looks like an injury or abrasion, and it may take a shed or two to fully heal. It may leave a bit of a scar however.
If there is anything sharp or abrasive in the enclosure, or a small space which it has tried to get into then retreated, or if it has been roughly handled (pulled backwards against some abrasive object), it will lead to an injury such as we see here.
Pretty much has to be an injury.
Jamie.