Some bots scan using giant lists of subdomains, e.g. https://github.com/danielmiessler/SecLists/tree/master/Discovery/DNS https://github.com/danielmiessler/SecL

Some bots scan using giant lists of subdomains, e.g. https://github.com/danielmiessler/SecLists/tree/master/Discovery/DNS https://github.com/danielmiessler/SecLists/tree/master/Disco.... Your subdomain may be on that giant combined_subdomains list, or perhaps some other lists that other tools use.