That's not exactly true, it depends on the architecture in use. For instance if using a Publish/Subscribe model, you could have had a service that listens to yo

That's not exactly true, it depends on the architecture in use. For instance if using a Publish/Subscribe model, you could have had a service that listens to your email being connected, and since the only reason to connect your email was to upload contacts, it would upload contacts automatically. Later when login with email was added, the same event was sent but whomever added the event didn't know it would case the upload of contacts. That doesn't mean it wasn't shoddy craftsmanship, bad architecture, bad QA and probably bad communication later on, but it could have been by mistake (at least at first).