It looks like it’s related to xmlrpc.php hits. There are constant spam attacks hitting xmlrpc.php, and it looks like each one causes wp-super-cache to clear cache files if “Cache Rebuild” is checked:
21:21:37 7816 /xmlrpc.php wp_cache_ob_callback clearing rebuilt files in /home/www-nxhm/blog/wp-content/cache/supercache/blogdomain.com/
More Details:
21:21:37 7816 /xmlrpc.php wp_cache_get_cookies_values: return:
21:21:37 7816 /xmlrpc.php In WP Cache Phase 2
21:21:37 7816 /xmlrpc.php Setting up WordPress actions
21:21:37 7816 /xmlrpc.php Created output buffer
21:21:37 7816 /xmlrpc.php wp_cache_get_cookies_values: return:
21:21:37 7816 /xmlrpc.php supercache dir: /home/www-nxhm/blog/wp-content/cache/supercache/blogdomain.com/
21:21:37 7816 /xmlrpc.php Not caching POST request.
21:21:37 7816 /xmlrpc.php wp_cache_ob_callback clearing rebuilt files in /home/www-nxhm/blog/wp-content/cache/supercache/blogdomain.com/
21:21:37 7816 /xmlrpc.php wp_cache_maybe_dynamic: returned $buffer