Google "Page Rank" Hiccup

I recently worked on a complete web site redesign project, converting the site from classic ASP to ASP.NET.  Many of the page URLs were going to change (despite the fact I kept the .html extension), so I whipped together a "redirection system" in the form of an HTTPModule.  The module simply checked that the file exists (or will be handled), and if not, queries a database to determine if the requested page has a "new" location.  It then sends a "301 Moved Permanently" response with the new URL (and logged the referrer so they can be notified of the new location, etc).  This system was meant for internal redirects only.  Of course, someone put an entry into the database redirecting an old page to an external page.

I then noticed the site has the #1 page rank on Google for keywords "acrobat reader setup," and the site redesigned is a university medical library.

There used to be a page named "acrobat.html" that gave basic instructions on the installation of Adobe Acrobat.  During the redesign, we decided there was no need for this page to exist anymore, and someone put in a redirect to Adobe's installation page (www.adobe.com/products/acrobat/readstep2.html).

The Google query: http://www.google.com/search?hl=en&q=acrobat+reader+setup

Google's cached page: http://www.google.com/url?sa=t&ct=clnk&cd=1&url=http%3A%2F%2F64.233.169.104%2Fsearch%3Fq%3Dcache%3APdCI1Oj1oNUJ%3Awww.hshsl.umaryland.edu%2Facrobat.html

My page (redirected to Adobe): http://www.hshsl.umaryland.edu/acrobat.html

If anyone has any ideas on how this happened (it has been like this for over a month now), I'd love to hear them.  I'm tempted to redirect a couple other pages to see if I can duplicate this.