I had a similar experience, so here's my hypothesis.
In addition to the biometrics, I also had requests for two additional documents. I got a series of emails saying my account had updates, which I believe corresponded to them uploading the biometric letter requests (one each for me and the spouse) and letters allowing me to apply for police certificates (two in total, a common one for each country addressing both my spouse and me). Maybe an hour later, I got an update for medicals. I believe that at this stage the home page actually still said application status was updated or submitted and messages read. Then I got another update email, and I believe this is when status changed to "Your action is required".
So my hypothesis is that there are periodic jobs that run, scan applications and change status. So sometime after the ADR and/or biometric requests were made, this background job saw I needed to do things and changed the status of the application accordingly, and triggered an email (either directly or that could be another batch process).
TLDR: You're fine and shouldn't think too much about these things until something is obviously wrong. And I should probably take my own advice