Nomura Bank was doing contingency planning for Brexit back in 2012. I'm really surprised if no one else did. For an article written in 2012, this is remarkably prescient:
http://www.thenewamerican.com/world-news/europe/item/12428-uk-may-soon-vote-to-leave-eu-despite-massive-pressure
I don't think the market dive was so much surprise as the markets traditionally not liking change (except in the rare cases where change seems obviously good for the market).
Also many drops are self-fulfilling disasters, as initial drops trigger trailing market-stops which in turn automatically drive the price lower as the holders of shares seek anyone selling to market out. There's actually good money to be made in it! -- I was stupid and bought into a stock I was looking at going up, because it had unexpectedly rallied after it should have dived down farther than it did after a dividend, then got caught upside down Friday morning. Should have sold short Thursday afternoon and rode the coaster down. :P (Fridays are often tepid or bearish anyway. I was galactically short sighted and it cost me.)