I have mentioned it here before about them testing something... and although we can't be sure, it might actually be the case here.
Actually this specific site experienced the same thing last January.
Couple of weeks before they roll panda update, this site tanked for all secondary keywords, including terms i ranked in google images, then after a week, it recovered and regain its previous rankings for every keyword including the keywords that ranked in Google images. Then after a week, they roll panda, all keywords that I'm actively targeting hit #1 and I ranked for so many keyword I don't even targeting.
March 6 came, Same site was penalized again but again not for my main keyword. Actually I still ranked #1 for main keyword and 2nd keyword that I'm actively targeting. But for my 3rd, 4th, 5th and those keywords that i suddenly ranked after panda 24 again tanked from #100+ to nowhere, these include the keywords that I ranked in Google images.
I'm actually hoping for the panda update since it might again bring back all keywords that i ranked before
As of now, i'm seeing slight increase of traffic but rank after march 6 is still the same.