Normally back tires wear out alot sooner than front tires. Often a front tire can last 2-3 times as long as a rear. So your problem is unusual. When you say the tire has a bulge, is the tire casing enlarging at some point, or is the tire lifting away from the rim? If the former and the bulge is a weakening in the casing, you either got a bad tire or it was somehow damaged. Hitting bumps or RR tracks when underinflated might do that kind of damage, or might cause a pinch flat. Also, check if your rim might have been damaged by the RR track. It might have a flat spot (not round if you look from the side) or the rim may be spread too wide at one spot. You would probably notice rim damage when braking. That type of rim damage could make the tire not seat properly and blow off the rim. Another bit of advice is to always get your butt off the saddle before hitting RR tracks, balance your weight between your arms and legs, and keep them bent to absorb the shock. Maybe you already do this, but occassionally we are all surprise by some road hazard and take a bump too hard.