Also known as age hardening. Check out http://en.wikipedia.org/wiki/Age_hardening. It is particularly prevalent in areas where there are large changes in temperature. Age (or shelf) hardening of aluminum changes it's bending characteristics and makes the section more brittle.

In a previous life I ran a company that curved and bent metals for all sorts of applications. One was aluminum bull bars for cars and trucks. A pack of ally tube that was open at one end and left outside for a month over summer was useless for bending even though it looked fine. The covered end would be fine but the end that was exposed to the weather would snap like a dry twig. Given time, the entire length would be the same even when it was covered.

Tiger Mike

p.s. yep - bored at work today!

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Precipitation hardening, also called age hardening or dispersion hardening, is a heat treatment technique used to strengthen malleable materials, especially non-ferrous alloys including most structural alloys of aluminium and titanium. It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice. Since dislocations are often the dominant carriers of plasticity (deformations of a material under stress), this serves to harden the material. The impurities, in fact, play the same role as matrix substances in composite materials. Just as the formation of ice in air can produce clouds, snow, or hail, depending upon the thermal history of a given portion of the atmosphere, precipitation in solids can produce many different sizes of particles, which have radically different properties. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called aging.