What causes muscles to become fatigued and sometimes develop cramps when you exercise too strenuously?

There are minerals which our bodies need (especially during strenous exercise)

called electrolytes - Sodium, Potassium and magnesium especially. These regulate functioning of muscles (think lubricant) and are run down during strenous activity of long periods of time while exercising. Its these electrolytes which keep your body performing at optimal temperatures and rates of efficiency, when this lubricant is depleted your muscles cant function smoothly and get stuck and knotted with the impending 'why now' reaction a cramp gives you! Its important to keep up good mineral nutrition prior to and in recovery from heavy sessions, to allow your body to perform at its peak condition, and during long periods (especially in warmer weather/regions) to drink sports drinks which include these minerals for this reason.

Muscles can also cramp, by going from one extreme to another. A classic example of this which not too many people will be familiar with is when cycling and descending a long steep mountain pass where you do not pedal hard much/at all for up to 30 minutes! when you get to the bottom of the mountain and ride up the next one 'cold' too hard at first you have a good chance of bringing on a cramp if your legs are also tired and full of lactic acid.

Lactic Acid:

The burn you feel during exercising is mostly due to the buildup of lactic acid in the blood. Lactic acid is a byproduct of strenous exercise, which begins to accumulate at a certain effort and then builds up faster than it can be flushed away at strenous levels.

During aerobic exercise this slow buildup of lactic acid over hours (or less with very high levels of effort) causes your muscles to fatigue and this is felt with a burning sensation in muscles used.