Server :: Abrupt Increase In Server Load Upto 300?

I run a server with nginx at the front and apache at the background. Nginx serves as a reverse proxy in here. As there were lots of DOS attacks, I have implemented Deflate-DDOS, APF and Nginx anti-ddos features and the server runs without a problem. Once every month, the load on server increases upto 300 and I receive an email like this :

Subject : Cron <root@server> [ -x /usr/lib/php5/maxlifetime ] && [ -d /var/lib/php5 ] && find /var/lib/php5/ -type f -cmin +$(/usr/lib/php5/maxlifetime) -print0 | xargs -n 200 -r -0 rm

Content :

find: `/var/lib/php5/sess_0ca40520ac8ecb66090746f90da17516': No such file or directory
find: `/var/lib/php5/sess_3839b0cbf042934183d56ff682c948e0': No such file or directory
find: `/var/lib/php5/sess_dec1ed8ea1f62caf7a42a29b9f82c506': No such file or directory

[code]....

There's more that 100,000 ( I suppose ) lines on this email. I have no idea what the problem is. It seems there's something with the session, but I really don't know what it is.