There are a number of things that can cause your tempdb to grow...but the most common are costly queries.

Here is another forum post with a similar question:

http://social.msdn.microsoft.com/Forums/sqlserver/en-US/0e710dcb-9046-41d0-a6af-be8e118ec7fc/tempdb-is-growing-abnormallyhow-to-check-and-find-out-what-makes-it-grow

That forum post references what looks to be a very good Microsoft article on the topic:

http://msdn.microsoft.com/en-us/library/ms176029.aspx

Hope this helps!

-Dane