The diameter of a Longneck gets larger as the level of the beer drops.
So, if you take a big swig, the carbonation is activated and the resulting bubbles that form start to produce a reaction.
The bubbles then try to expand from a larger area - the larger bottle diameter under the neck, into a narrowing area - the neck itself - and if you tried to suff 5 pounds of sand in a 3 pound sack well here's my take on it......the rule we learned in Mr. Ralf Pettit's 11th grade Physics class, "no two objects can occupy the same space at the same time", hold true here,those bubbles have to go somewhere, and a smaller area to occupy means that they go all over you or all over the bar.
Now if someone can tell dumb ass me, why "very cold" beer produces less bubbles than just "cold" beer does, my night will be complete.
[:D]
.