I wondered why I always lost the tail first, until the first time I actually looked at the tail turret.
FWIW, in my experience, only the Il2 loses the rear gun as quickly. You can fire heavy cannons into the tails or in the proximity of gun positions on other bombers without result, but a burst into these 2 is almost certain to knock them out. Given their construction, and lack of any protection, it makes sense, I suppose.