In your Keyword ranking report you can take a look at the SERP that SEOmoz saw when they collected your ranking.
Click on the little pie/clock shaped icon when you hover over your search term or click on the drop down and select ranking history.
Note that there's a comment that says:
"We gather rankings that are non-personalized and independent of search history or location. This method has provided results showing what most users see most of the time, though fluctuations can and do occur."
If you look at this snapshot, can you see what else has moved about?
#3 to #40 does seem like quite a large fluctuation, but I've seen the same thing on occasion too and I've used the keyword difficulty tool to double check too.
Remember that SERPS aren't static and depending on your niche can be subject to quite a bit of fluctuation.
Did you notice any changes to search traffic? Take a look at your google analytics over the affected days to see if there was a drop one hour to the next... Was there anything else that might explain this such as server downtime or your site being unavailable etc?
Keep an eye on trends rather than individual results but yes, do check out any surprising changes to the position of your priority keywords!
Hope this helps.
