If you're using Discords mobile app, their latest update now also automatically opts you into contact finding. If you and someone else on your phones contact list has it enabled, it will automatically connect you two. This could potentially be a major privacy or safety issue.

To fix this:
1. Open the mobile app
2. Click on "Messages" on the bottom
3. Click on "Add Friends" at the top right
4. Uncheck "Allow contacts to add me"

At the time of posting, doing that will actually re-check it once you leave the screen. A potential workaround to stop this:
Go to Settings -> Privacy & Safety -> Scroll down and there is a "Sync Contacts". Turn that off.

Also for safety, in the settings area previously mentioned there is also a "Discovery Permissions" for "Phone" and "Email" which allows discovery via that. You may wish to have those disabled too.