As for active torpedoes, they home on pings. I don't think such a tiny object as a torpedo provides much return on active sonar, and there may be a discrimination range set by the producer.
As for passive ones, I THINK that a plausible explanation would be frequency discrimination. If not, then it still takes a hell lot to make a torpedo HIT another torpedo. These weapons are intended for enormous objects, and they are not guaranteed a hit within two-three feet.
As for detonation causing a wave that would put other torpedoes out of the game, you have to remember that not only does a pressure wave have different ways of moving underwater, but 260kg (very few warheads are larger. I haven't seen a single torpedo warhead that's a TON! If you have, I must be told so I can use it!
) is not all that much, especially not when much of the blast is given off to the submarine.
Use mines as reference. These are often quite devastating, the heaviest in DW being a metric ton. Now, when these explode underwater, the effects are not quite like one would expect them on land. The explosion would visually take the shape of a ball, but seeing as water is much heavier than air, and growing heavier with the increasing depth, an explosion is given very little space to move about. The water, while certainly being pushed outwards, will constantly put its weight on the explosion, limiting it dramatically.