We seem to get them a lot over in Guernsey although not quite to that scale!!
It seems that the overfalls just off Guernsey seem to somehow create them, maybe wind, tide, sea state etc play a part and then push them out to sea.
I've been trotting along quite happily half a mile past the overfalls on an almost flat calm day and then out of nowhere a wave of around 10-12ft comes hurtling past!!
Quite unerving the first time it happens and your not expecting it! At first I thought I must have caught the wake of the Condor Wavepiercer but no other boats anywhere near.
I've had this four or five times during the past three years.