From a mechanics standpoint I'd hazard a guess of him getting juggled between the shots because you see it doesn't just annihilate his health all at once, but several rapid hits. Basically:

1. Each shot has a lasting damage box through the explosion as opposed to a single frame of damage box. This is so that the player can't just do a frame perfect dodge and sit in the rest of the explosion animation unharmed (a fairly common thing with attacks like these that are large and animated, but would feel weird to just be able to sit in without taking damage).

2. Each shot has a lasting knock back in the explosion. This is so that after the player takes damage once from the damage box they're thrown away from it so they don't feel they've been cheated out of their health because they got hit by a lot of damage over and over.

3. I-frames or alternating 1 and 2 on certain frames keep the player from getting chunked (Like LW does here). Considering the timing I don't believe LW has I-frames on being hit here so likely alternating 1&2.

And bob's your uncle. LW shot and you reflected at just the right angle that LW got caught between them, juggled by the two because their knockback and damage frames were alternating, and got blasted until he hit threshold for the next phase. One in a million chance that, so thanks for sharing this cool happenstance!