You might have been hit by an aggressive bot (or more), that were scanning your site(s). Have a look at the web server logs and if this is the case you might want to block some bots with something like this in your .htaccess file(s):
SetEnvIfNoCase User-Agent .*rogerbot.* bad_bot
SetEnvIfNoCase User-Agent .*exabot.* bad_bot
SetEnvIfNoCase User-Agent .*mj12bot.* bad_bot
SetEnvIfNoCase User-Agent .*dotbot.* bad_bot
SetEnvIfNoCase User-Agent .*gigabot.* bad_bot
SetEnvIfNoCase User-Agent .*ahrefsbot.* bad_bot
SetEnvIfNoCase User-Agent .*sitebot.* bad_bot
<Limit GET POST HEAD>
Order Allow,Deny
Allow from all
Deny from env=bad_bot
</Limit>
You can add or remove bots as needed.