Many things can cause your server to use a lot of bandwidth.
1 - You may have greatly underestimated your usage. It is wise practice to always purchase a server with more than double what you estimate to use. This will ensure no bandwidth overage charges.
2 - There is a script or program that is "stuck". If this is the case you would want to check the server's running processes and "kill" the process.
3 - The server has been compromised, someone has "hacked" into your server and is using it for their own purposes. Please ensure your are running a firewall and root check kits regularly on your server. If this is the case, please notify us ASAP.
If your bandwidth is graphing is showing extreme values please submit a support ticket and we will look into the issue.
A number of processes or bad programming can cause rampant bandwidth usage. Usualy we will track the process down to kill it ;)