MSN seems to do this about every two weeks. I just experienced the same thing. Usually MSN will place more weight on keywords in your domain name than the other SEs and sometimes on of their algos will skip your site description and domain name all together entirely. It can be frustrating. Best bet to weather these types of storms is to make sure that your keywords are sprinkled well at different spots on the page and that your domain name and page name extensions is keyword relevant. I usually try to only optimize each page for only one keyword phrase at a time and this seems to help me stay towards the top no matter what algo they are using.
Something definitely happened.. I track about 300 keywords for one of our sites, and I'd say about 75% of them have had their rankings just drop like a brick.. We're talking from #7 to #183. Many of these pages haven't been touched in awhile, so it wasn't something that we did to cause the drop. Since everything we do is white-hat, there's nothing we can do except wait and see if things recover. Really strange, to say the least.