Why does Byleth do that

I had a couple of matches with this good Byleth player in elite to work on the matchup and it was mostly successful for me except for this one game where I was on last stock, got up b-d into down air and died off the top at like 60. Like I was pretty sure I was DI-ing out but it's still insane that you can just die at 20-30 from poor DI on that broken ass move