Short, non-technical answer: These waves are actually the "sum" of a number of different waves (of different frequency and/or phases). It can happen that many

Short, non-technical answer: These waves are actually the "sum" of a number of different waves (of different frequency and/or phases). It can happen that many of these _usually_ cancel each other out in the sum, but once in a blue moon, their peaks happen at virtually the same time. Then you get one massive pulse. The moments right before and after could look quite normal.