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Why does the Brexit cause a fall in crude oil prices? 
Uncertainty has very far reaching effects.   Oil is up ~100% since February and down ~40% from it's 52 week high (and down even more on a longer timeline).  It's not exactly a stable investment vehicle and moves a few percent each day on basically nothing.  A lot of securities will be bouncing around for the next couple weeks at least while folks remain uncertain about what the "brexit" will actually mean.