This is something that is known, but not much research has been done into it. It's generally been thought that on heavy characters at low %s, the tipper upsmash combos into the weak hit upsmash because they aren't sent high enough to avoid the second hit.
Easy way to mess around with it is on tall chars at 0. Bowser is easiest.
This is the first time I've heard of it happening at high percent though.
I'm not sure how much DI affects it.
In theory the way this works is:
Upsmash tipper hit is above sheiks head, weak hits are beside her. Opponent has to be heavy enough, at low enough %, and be already low enough (so best for them to be grounded) to still be in range of the second hit upsmash. I guess upsmash being stale would help, as well as maybe hitting something else so you're in more hitlag than the opponent (e.g. being on duck hunt and hitting a duck and the opponent at the same time with tipper upsmash).
Bowser jr has a weird property where he takes less % and knockback than normal when he is hit on his koopa kart, and more % and knockback when hit on his body. Though I feel like this would affect it more negatively than positively because for him to be grounded and the tipper to hit, you'd be hitting his body.
Can I ask what the situation was where you hit him? Was he grounded or in the air? Was he holding a mecha koopa, or was one near? Had you used upsmash recently before?
Again this is something known but not researched much, and I've never heard of it happening at high %s like this, always thought it would just be a low % thing. If you have the replay could you record it with a camera or something, so I can see more about it?