Is this a very recent change? I've never seen this behavior before - today, I hit 'commit' in Source Control. Rather than it then the button changing to 'sync changes', and waiting to push the changes to repo until I hit that button, it automatically synced to my GitHub repo.
When did this change? I can't find anything about this on the VSCode github or release notes. This seems like a massive thing to change by default.
You can modify this behavior in your settings.json file by altering the git.postCommitCommand setting.
The setting calls an action after a commit:
"none" – No additional action (the "old" behavior)."push" – Automatically pushes your commit."sync" – Automatically runs a sync, which both pulls and pushes.