I know that the lower the pH the faster fermentation occurs. Why does this happen?
pH affects the shape of proteins. In the case of fermentation a collection of enzymes is responsible for the metabolic processes that occur. - an enzyme is a protein which performs a metabolic process. For example sucrase is an enzyme which breaks sucrose down into fructose and glucose. They are kind of like organic catalysts.
Proteins are made of amino acids strung together in a long line, based off DNA. The amino acids bond up into a long polymer. Then cross linking occurs between the R groups on the amino acids. This makes the unique shape of the protein, because the cross-linking turns the protein into a 3-d shape.
Amino acids may be acidic amino acids, or basic amino acids, due to the R group on the amino acids. If the pH is increased, this affects the shape of proteins, by disrupting the bonds in the protein. In the case of fermentation, you say the rate increases when it get's more acidic - when the pH is lower. This is because the organisms - the yeast - producing the enzymes to ferment glucose, have adapted to acidic conditions. This means that through natural selection the enzymes today have been selected because they work best in acidic conditions, which bend the protein into the correct shape to allow fermentation to occur.
In this article it says that 10 enzymes turn glucose in pyruvic acid:http://en.wikipedia.org/wiki/File:Pyruvic-acid-2D-skeletal.png
And then two enzymes turn this into ethanol and CO2. The last two enzymes function best in acidic conditions, presumably because they have evolved to work best in the acidic conditions made by the pyruvic cid. However all fermentation stops after the pH drops below about 4.2. The optimum pH is about 4.8 - 5.0. After this we can assume that the pH bends the proteins out of shape too much, and the protein is said to have been denatured.
So to conclude... The rate increases because enzymes are all suited to specific conditions, dictated by natural selection - the conditions they have evolved around - and in the case of fermentation the last 2 enzymes - turning pyruvic acid to ethanol and CO2 - work best in acidic conditions, as they have evolved to those conditions. But increase the pH too much... and the protein denatures, and fermentation stops completely.
In addition to the other answers, another reason for the increase in fermentation rate in a lower PH is that the reaction actually uses up more protons (H+) than are actually produced in glycolysis. Like lactate fermentation the 2 NADH and 2 H+ produced during glycolysis are converted back to NAD+, but in ethanol fermentation there is an earlier step in which a hydrogen atom is added to the pyruvate when it is decarboxylated.
Take a look at this image. The "acetaldehyde" (ethanal) gains 2 hydrogen atoms from the NADH and H+ which is expected, but it has one more hydrogen atom than the pyruvate molecule. Now look at this image of the mechanism of pyruvate decarboxylase. See how in the third part an H+ appears out of nowhere?
In low PH solutions the concentration of H+ is greater. This increases the rate of reaction which, of course, increases the rate of ethanol fermentation.
I don't know that it's correct to say that lower pH increases fermentation rate in all cases. It is true for pH ranges that are close to neutral, but there is a point where increasing acidity will kill the yeast.
In general though, yeasts and molds are more tolerant of acidic conditions than they are of basic conditions, down to a pH of about 5. Bacteria, on the other hand, tend to not be very tolerant of acidic conditions (at least, not the bacteria commonly found in places where we are trying to encourage fermentation.) This article compares the growth and fermentation rates of bacteria to yeast under different conditions, and finds that under mildly acidic conditions (pH of 5.5 to 5.0) the bacterial growth rate was slowed and the yeast growth rate was maximized. Since bacteria competes with yeast for food, the ideal pH is the one that kills the most bacteria without slowing the yeast growth rate down. A pH of 5 is about the same as a cup of black coffee, and is about one hundred times less acidic than orange juice.
I also found this article which describes another interesting effect - the bacteria that was used in the first experiment actually starts producing ethanol as a fermentation product under acidic conditions, instead of just lactose. I suspect that growth rates are a bigger factor, but it is an interesting complication.