Dolphins are meant to chase after players in boats, so it makes sense they start speeding up, but the speed they increase to at the moment is crazy, they can't even chase the boat because they are too fast and overtake you at around 10× your boat's max speed.

Looks funny when they dive while in super speed mode, saw one jump like 20 blocks high. Place a bunch of dolphins while you're in a boat in 3rd person, then move forward a little bit and watch them go crazy.