It is caused by an aircraft being more directionally stable than laterally.. For example a lateral disturbance causes the plane to bank, the nose will then tend to follow the direction of the bank which in turns increases the bank (as the aircraft is not as stable laterally). If nothing is done, the aircraft will end up in a steep spiral dive.
The opposite effect (when the aircraft is more laterally stable than directionally) is called Dutch roll..
Last edited by MONSIEUR; 15th February 2008 at 08:10.
Reason: Apologies.. I have now replaced "Longitudinally" by "directionally".. it should make more sense now!.