Unknown contacts appeared out of thin air
There are many possibility:

1: Facebook sync with your contact info, as I know, Facebook will suggest people you may know to you.

2: Are you using a same Apple ID with your families or friends? Contacts will sync to your other device with same Apple ID.

3: Are you using other's SIM card? SIM card store contact inside the micro chip!!

4: Your Apple ID have been hacked.....change your password immediately and never tell others your ID and password.