In shared web hosting, the ability to use your account is limited not just by your space and bandwidth, but also by how much of your server's resources (processor and RAM) you may be able to use at a given time. Since you are sharing a server with many other clients, each must operate their site within certain limits to ensure that all accounts perform as well as expected.
We also completely understand, however, that webmasters are often unaware of instances where their site is using too many resources, and that such usage is not done maliciously. Our staff is partially dedicated to handling resource usage issues and working with clients to resolve them.
For a more detailed explanation of our usage policies, please see this page.