Well you said you got him down TO a quarter health. If that's the case, one person was late. 3 out of 4 people were on time. Hence, you did 3/4 damage. If you only did a quarter damage to him, the one person was early, thus his bombs destroyed everyone else's. 1 out of 4 sets of explosions, meaning you did 1/4 damage. Either way, no bug, no glitch, just poor timing.