It may be many single threaded requests are running concurrently, or some requests are running with parallelism.
A table scan will most likely cause parallelism.
Check you CXPacket waits stats.
You may be in need of either an index, or a change to the code so that it can use an index or at least let you know that it needs an index.
Catch-all queries done right [/url]
Gail Shaw's Performance Blog[/url]