When a surfboard takes off it moves through three phases. Initially it pushes water in the displacement phase. Vessels must push water out of their path to move. Following the displacement phase it goes into transition. This is just before taking the drop. The surfboard starts to lift and pushes less water. In the final phase gravity takes over and the surfboard planes on its aft half with forward half above water. It is planning hydrodynamically and not pushing water with its nose. The flat bottom works well with gradual acceleration.
In modern surfing with vertical maneuvers and flight, the surfboard skips the transition phase. The movement goes from displacement to flight, in late take offs. In a free fall landing it goes from planning to displacement. When the transition phase is skipped, a flat bottom surfboard may stop. When the landing is perfect, the flat bottom will stop because it pushes water forward. Water builds forming a bow wave, and blocks movement with hydrodynamic resistance. The flat bent underside creates lift and pushes the board back. A round or Vee bow pushes water aside, out of the surfboards path. This cushions the landing so the surfboard can go into transition and possibly complete the ride.
Warning, the following video is tragic and very, very sad. Science can explain the formation of waves. Science can also explain how a surfboard moves.