It seems it was an unintended consequence of the Google Maps change and many users were affected and confused by it [1]. If you add a Billing account to a cloud project, Firebase will automatically upgrade you to Blaze. Since Google Maps added a billing account automatically to your Google Cloud account, that auto-upgraded your Firebase plan.
The response from the Firebase team [2] was that no additional charges would occur based on this incident, and they've opened an incident report [3] in their dashboard.