Torpedos have an arm delay to prevent damaging the ship or plane that dropped them into the water.
When you get down low and drop a torpedo with no arm delay, it can explode on impact and the shockwave and splash and destory the plane that dropped the torpedo by causing the tail of the aircraft to be forced up, causing a nose down angle of attack. Which at low altitude can be too much to recover from, especially since the shockwave more likely damage control surfaces and plane hull integrity as well.