It is a common brute force attack technique to run author scans on a WordPress site. It is used to crack passwords for the known usernames. You can prevent these attacks by blocking author scans. The code snippet for this is:
# BEGIN block author scans
RewriteEngine On
RewriteBase /
RewriteCond %{QUERY_STRING} (author=\d+) [NC]
RewriteRule .* - [F]
# END block author scans