When you'll drinking and/or got alcohol in your system, your liver can't to replenish the liver store of glucose, it can't start doing this again until it has processed all the alcohol out of your system on average it takes your liver an hour to process one unit of alcohol...
So when you'll drinking to start off with, the alcohol doesn't make much difference or your BG may raise quite sharply depending on what you'll drinking (if it's got carbs in like beer, larger etc) as your liver is still dribbling glucose into the system, but once the supply runs out, then your BG can take a nose dive and increase the likely hood of a hypo, also until the liver finished sorting out the alcohol, your liver isn't being replenished, as even when it starts to replenish it takes time for the glucose to start to be delivered to your system, during this time you are not only at a high risk hypo, but you have also lost your safety net of the liver dumping it's glucose store to bring you out of an hypo.. It also means that a glucogen injection won't work.. As glucogen is the hormone that's needed to single your liver to dump it's store...
You need to test frequently while drinking and for a while after, some good advice for carb up take before going to sleep etc.. But you should also insure that you don't drink alone, your friends are aware that you'll diabetic and any behaviour that might indicate a hypo, needs to be acted on and if paramedics are called that they are aware that you've been drinking...