Sounds like hard trimming.

This covers this issue and provides a link to further information

http://blogs.msdn.com/psssql/archive/2007/05/31/the-sql-server-working-set-message.aspx

It can also - from what I've seen - happen if there's a problem with your hardware, check this out as well, particularly if this is a recent occurence on a box which has been running fine until now. In this case, you may be looking at moving to other servers - fixing the hardware and moving back