sync – Super old email contacts suddenly appears in my phone contacts

When I woke up this morning I suddenly had 6 new contacts added to my contact book on my phone. All of them were emails, some of them I recognize from WAY back but I sure haven’t had contact with them in a LONG time. I guess they were added in some kind of sync activity, but I’m pretty sure none of these addresses that were added, is featured in any of the accounts I’m currently using (gmail, yahoo) etc. I had contact with these people way before this

Why did this happen?

Is there any way to find out where a specific contact were synced from?