I have received many "friends" who have accepted my request when I don't know them, let alone requested their friendship! What are the possible explanations for this? Am I getting hacked? This has happened numerous times.
The only possible explanation of this is that someone had/has access to your account and sent those friend requests to random people who are completely unknown to you. Yes, surely you are/were hacked.
You should also install this Google Crome extension, with which you can see the list of people you have sent friend request and not received any reply yet: https://www.unfriendfinder.com/news