Pitch-Up

pitch-up is a severe form of stall in an aircraft. It is directly related to inherent properties of all swept wings, and seen primarily on those platforms. Unlike conventional low-speed stalls, pitch-up can occur at any speed, and are especially dangerous when they take place in the transonic; at these speeds the aerodynamic loads can become so high as to break up the aircraft.

It can also occur at low speeds, in which case it has been called a Sabre dance, a particularly dangerous behaviour of swept wings that become apparent.

But how the pitch-up happens : 

As the aircraft angle of attack increases beyond a certain figure, the aircraft continues to diverge in pitch, thus increasing the angle of attack even further.  

as we know before ; this tendency normally is found in swept-wing aircraft mainly because of wing-tip stall, which makes the center of pressure move forward. The wing moment is reduced and a nose-up pitching moment results, which aggravates this tendency. In straight-wing aircraft, wing-tip vortices can cause pitch-up if the aircraft has a low aspect ratio and short-span wings.