Aerodynamics.
When an airplane is in flight, the force of lift acts on a point along the aerodynamic center (a line extending from the wingtip to fuselage) near the wingtip. This lift induces a moment around the elastic axis, a line parallel to and behind the aerodynamic center. This produces a slight horizontal twist in the wing, which increases the angle of attack, which in turn increases lift, inducing a larger moment about the elastic axis, and so on.
At relatively slow speeds, the torsional stiffness of the wing is enough to counteract the twisting. However, the moment increases as the square of the speed, but the stiffness remains constant. Therefore, at high speeds, the wing fails - sometimes causing it to twist right off of the plane.
The speed at which the wing stiffness can no longer counteract the twisting moment is called the "wing divergence speed".
In order to counteract wing divergence in high-speed aircraft, wings can be designed to minimize the distance between the aerodynamic center and the shear center (on the elastic axis). This is usually accomplished by means of either swept forward or swept back wings.
Swept back wings induce a downward twist in the leading edge of the wing (because the moment arm from the shear center to the point of lift on the aerodynamic center extends toward the back of the plane), which reduces lift, counteracting wing divergence. Most high-speed aircraft have swept-back wings.
Sweep forward, on the other hand, induces an upward twist (because the moment arm, in this case, extends toward the front of the plane). This exacerbates wing divergence - exactly what we don't want. Only a few experimental craft use forward-sweep, presumably in an effort to increase the effect of the ailerons and thus increase maneuverability - but that's a tale for another time.
Here's some bad ascii art, in case it helps you visualize this better. "ac" is the aerodynamic center, "ea" is the elastic axis, "x" marks the point where the lift acts, and the diagonal line that's trying to become an arrow represents the moment arm.
/ /
/ fuselage /
_____________________/ with /
/__x________ac______ / swept /
/____\_______ea_____ /___ back /
/ \ / / wing /
/__________\_________/ / /
\ / / /
\ / / /
\/ / /
/ \/ /
/ V /
\ ^ \
\/\ \
/\ \ \
/ \ \ fuselage \
/ \ \ with \
/ \ \ swept \
/ \ \ forward \
_____/_______________\ \ wing \
\___x__________ac_____\ \ \
\_____________ea______\_\_ \
\ \ \
\_____________________\ \
\ \