claaarky - 9:46 pm on Jun 26, 2012 (gmt 0)
Algo change - they've changed the algo
Panda refresh - they've fed the latest user metrics about all our sites into the algo.
Panda and Google crawling your site could be related. User metrics collected about your site since the last Panda update (assuming that's the period they look at) may contain pages that don't exist on your site any more, so maybe they may need to check they are still there before they can apply the Panda score to all your pages, which the algo will then swing into action and rankings will fly all over the place as improving and worsening sites fluctuate.