18 May 09
01 Jun 09 9:23 pm
Google does over 300 "updates" each year (quoted from Matt Cutts, sorry don't have a link on hand) so maybe
a factor of your site was looked on as unsavory by a minor update - there are many, many reasons why a SERP can drop down. Looking over any changes made over the last few weeks is a good start. The problem is that unwinding them all and waiting to see if your site is ranked again is a timely process, as well as the fact that you will lose all these features that you've added in for a reason!
At an educated guess - I would suggest that google has observed something unsavory about this page. Has the drop been site-wide or just isolated to this page?