A bacterial infection is the most likely cause of death, but usually you don't get this kind of bacterial infections if there is not another problem going on with the tank.
I have a pair and there is a quite a bit of chasing going on but none of them has ever ran into a rock or coral. These guys are pretty good at snaking around sharp edges at high speed.
But for about a month, right after I put her in the tank, the female got an infection (possibly Uronema) and developed a big hole in her side. I had her already on the loss list but she hung in there and it healed completely. Based on that I would say, if the conditions are good they can handle a lot but they are quite sensitive to bad water quality and high bacterial loads.