Transients are an inescapable feature of wave train interference. They can be caused by two trains of comparable frequency intersecting at angles, or by parallel trains of differing frequency. The transient can be a one-off event at a single location -- like right under your boat -- or it can 'travel' as the constructive interference pattern is reproduced thru the matrix.cross waves and/or current (oh yea, and/or odd bottom contour - even in reasonably deep water) are the typical situation . . . but (going a bit far afield) there is also some quantum mechanics theory suggesting that even without those factors individual waves can steal wave energy from the surrounding wave field and become 'rogue size' (eg more than twice significant height).