Okay, so. My friend and I were doing Craglorn delves. We went into a delve and caught up with a guy who had been killing all the mobs in a NON-group delve, it was clear up to him. We started killing mobs alongside him. He kept pace with us, or we kept pace with him, either way just casually trotting along killing all the mob groups we encountered. The last room before the boss, we go up to the mob group outside it, and he hangs back for a second until we two have engaged the group, and then he hits the accelerator and sprints superfast down the rest of the hallway into the next room and starts burning down the boss. My friend has ranged attacks and abandons our fight to race in there and manages to get a last-minute distance tag on the boss.
So here's my question. Is there any mechanical reason why someone would want to kill the boss by himself? Is there an achievement for soloing delve bosses that I've somehow missed? I would've just thought it impatience if he had skipped ALL the mobs and just let them chase him through the delve (like the next guy we encountered) just to get the thing done, but he didn't. He fought at a regular non-sprinting pace, waited a moment to see that we were busy with the last group and then he ran as fast as he could ahead a pretty good distance to go after the boss. I'm baffled, and looking for some kind of explanation other than what it looked like (that he was deliberately being a you-know-what just because he could).