There might be couple things which can cause your website exceed your monthly bandwidth.

1. You may be exceeding your bandwidth limits due to high volume of visits to your website as specially if you have your website image and media intensive.

2. Your website might be attacked. This means that someone could be sending hits to your site with a program, to use up its bandwidth so that site will be unavailable. (This is called a "denial of service" attack)

On cPanel you can investigate your bandwidth usage by accessing 'AWstats'