Justin, pH drops because of the introduction of an acid. Acid can come from CO2 (carbonic acid), decaying matter (hummic and tannic acids) and animal wastes (ammonia). Nitric acid is also a product of nitrification, so in reality, your pH is constantly dropping. What you have in your favor is alkalinity; alkalinity buffers (hence the term buffer for alk products) the drop in pH by (very simply) donating an OH (hydroxide) molecule which binds with a h+(which is an acid) and effectively neutralizing it. The reason we as aquarists keep our alk so much higher than natural seawater is because we have so much more acid to "buffer".
So when you have a high alk, you tend to have a more stable pH. Of course if you have a high CO2 concentration in your home, or you spill vinegar in your tank, you may still see a drop in pH. Simple, huh?
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Brad
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