Rogue wave

Longer period waves travel fastest and therefore overtake shorter period ones. Also there may be several wave trains present that have come from different wave generating areas. As waves pass each other they may become 'in phase', 'out of phase' or somewhere in between. When two waves are in phase their wave heights are added together to give the combined wave height. When they are out of phase they are subtracted.

Clearly if you have three wave trains with wave heights of 2 feet and they were momentarily in phase, the wave produced would be 6 feet in height.