The effect of alcoholic beverages on blood sugar vary over time. First, alcoholic drinks usually also contain lots of sugar and other crabs as well. Same as eating candy, these carbs will be absorbed by your gut and released into the blood stream, where the blood sugar content therefore rises. I guess the 'spike' refers to this.

However, while that is happening your liver starts to process the alcohol and break it down. From approximately a BAC of 0.045, your live is so preoccupied with the alcohol that this seriously interferes with its ability to release carbs into the blood stream on demand; one of the many jobs the liver has is storing sugar in order to release it at short notice.

Under the influence of alcohol it can't do that, resulting in a lowering of blood sugar some hours after consumption.

Type I diabetics - the ones who can't make any insulin of their own and have to inject it - are at particular risk from this mechanism and have to be very careful with alcohol.