My doctor told me it's because the liver is busy trying to process the alcohol, it doesn't release any and/or very little stored glucose, which can cause hypoglycemia, especially if you haven't had anything to eat. I don't have this problem with beer, but hard alcohol will drop my number in nothing flat and it stays low for hours afterwards.

I did find this online: http://www.medicalnewstoday.com/releases/9314... that has a more scientific explaination.