@gehrlekrona - Exactly the same dates as me, same percentage drop, my domain age is around 10 years. I'm dropping bricks here.
I also noticed some weirdness re: my sitemaps. I'm not sure if that was a personal issue with my url capitalisation methods.
Did anyone else noticed a massive drop in index presence shown in WBMTools? (it's recovering now, 1000 urls an hour)