Not all the waves occur with the signs known to us. The above video shows the occurrence of a Rogue wave.
Rogue Wave, also known as the monster wave, forms even out of thin air and has no warning of occurrence. On its arrival, the waves reach a height of 80 feet and are a threat to the sailors as well as ocean liners.
It is an open water phenomenon far larger than the average large wave. Different from Tsunami, it is defined as the mean of the largest third of waves in a wave record.
Rogue waves seem not to have a single distinct cause, but occur where physical factors such as high winds and strong currents cause waves to merge to create a single exceptionally large wave.