Getting multiple unused accounts suspended like that usually happens when you log into many accounts on the same device. You should be using antidetect browsers (like Hidemyacc, Gologin, etc.) to keep each account in a separate profile for safety. If you've already separated the logins and still got bans, the issue might be your IP, it could be blacklisted.
But if you're sure the environment is clean and you're using fresh IPs, then Facebook might just flag your accounts as suspicious. Even staying inactive for a long time can be a red flag these days