Question about `github.event.before` in case of a `push` event

In some cases, the value of github.event.before is 000… This usually happens when I push a new tag (and I think a new branch).

Can someone tell me why this happens? If I know exactly when and why this happens I can better write my script.

Here is an example: Use new action · kartikdc/commit-msg-action@8e0c70b · GitHub