Suppose we’re working on a PR and push a change to the PR’s branch: that’ll trigger the Check change set workflow. Now, if we push multiple times in quick succession then every push will trigger that workflow and suddenly we have that workflow run concurrently multiple times — even though, technically, only the most recent push is required.

This change automatically cancels existing workflow runs (if any) upon pushing to a PR’s branch.

For more details, see the docs.