Adding to what has already been said- New registrations are the food for probing. You could have an IP on someone's naughty list from a previous user. It's any other day on the internet. That's just what the neighborhood ( the whole internet ) looks like from the sidewalk. So, Set up a free Cloudflare account, move your DNS of record to them, and run traffic through Cloudflare to your server.
Since the author is not actually experiencing any issues, they're just curious, there really isn't any need for this.