There is an anesthetic quality to alcohol, and an initial uphoria that could serve to make a full bladder easy to ignore at first. A full bladder can make it harder for the kidneys to drain more urine into the bladder. Once emptied the kidneys would find it easier to fill the bladder. I knew a heavy drinker that explained why he would stand at the urinal elbows sticking out, hands pressing on his back... "to empty his kidneys as well as his bladder".

When the liver breaks down alcohol, one of the chemicals, acetaldehyde, I believe, is an irritant to the nervous system and could explain why, after steady drinking over several hours people actually seem to wake up. In this state they are more aware, and with beer adding more liquid... the floodgates are fully open.

One hoser stood peeing into the woods. Lifting a bottle of beer to his mouth he said, "ironic isn't it".

Alcohol is a diuretic, chemicals that act on the kidneys to reduce the amount of fluid reabsorbed by the nephrons (thin cappilaries).

Alcohol proces this diuresis effect via inhibition of Antidiuretic Hormone, ADH (AKA vasopressin). In a normal situation ADH is released from the pituitary gland (located in the brain) and causes the nepherons to become more permiable meaning more water is able to pass back into the blood stream via the nephrons. By blocking the production of ADH you are reducing the amount of water able to pass back into the blood stream (causing dehydration) and therefore you produce urine faster leading to the bladder filling up quicker, meaning you need to pee more often. This process takes time to kick in and therefore you dont immediately feel the need to pee but when you do eventually go 'openining of the flood gates' you can't stop going as the ADH levels have had time to decrease.  

This dehydration method is also behind what leads to some of the effects of your hangover. So remember to drink plenty of water when you get in ;-).

sorry not an answer but just wanted to metion that here in oz we call it 'breaking the seal'. I would have friends who would sit there for ages needing to urinate but would put it off in fear of 'breaking the seal'. However in light of the answers given here i can only assume that this would make the effect worse?