Today's Brief Lesson in Basic Aerodynamics 101:The main thing wings are designed to do is generate lift. Faster-moving air is less dense (lower pressure) than slower-moving air (higher pressure), so a wing is shaped with an outward curve on top, and a (basically) flat bottom. This makes the airplane get "sucked" upward by the lower pressure of the air above the wing.
Another thing wings are designed to do is maintain something called "laminar flow," where air moves over the entire wing smoothly. (Fig. 1a)
When a wing is tilted upward too much, and/or the plane's not going fast enough, the air flow begins to separate, like an axe splitting wood. (Fig. 2) Instead of the upper surface of the wing contacting lower-pressure smooth air, it's creating turbulent air (Fig. 1b) that keeps the airplane from going up any more, and begins falling. This is called a "stall," when the airplane stops being a flying machine and instead becomes a hunk of metal several thousand feet in the air with some poor bastard in it.
Fig.1
Fig.2
When an airplane that's not designed to go supersonic does it anyway, or gets close enough to it, something similar happens. Even though the airplane and the air below it isn't passing by at the speed of sound, the air
over the wing is, since it's moving faster already. Air stops acting like a gas, and instead acts like water coming out of a hose with a nozzle at the end--it doesn't so much flow over a surface as much as it hits it and goes flying off in another direction,(Fig.3) making a shock wave (Fig.4)
Fig.3
Fig.4
Again, the air isn't making contact with the wing like it should, and again, the plane drops out of the sky.
Long Story Short:What Fly320s said, but with pictures.
