More than likely someone with a high traffic site fat fingered an IP address and sent all the traffic to you.

I am curious if you were able to capture the headers at all. The 'Host' header would allow you to figure out the site these people were actually trying to hit. I'm fairly confident it was not your domain. You may want to to put the Host header in your access logs. This may also explain why everyone got a 301 and using http.

No matter what you should be able to see this header 'Cf-Connecting-Ip' to know the true source.

While cloudflare is somewhat masking the origin IPs, a similar mistake without cloudflare would send at least the amount of traffic towards you.