The definition of object has changed

SQL 2008 R2

I have a sproc that runs daily. Every week, at least once, I get the error that "The definition of object 'xxx' has changed since it was compiled." The sproc hasn’t changed in months.  The underlying tables haven’t changed in months. When the error happens we re-start the job at the failed step and it works just fine. How can I determine why this is happening?  More importantly, what can I do to make the error go away?

André