It might appear that way, and it might actually be a bug, but I play a decent amount of Malzahar also and unless you can can reproduce it, I'm inclined to think it was just good timing.
There is a delay on both ultimates activating. If ori saw malzahar closing distance to ulti, she could send the ball to his feet and hit R. While she is doing that, because she stops moving to cast, Malzahar gets into ulti range and starts his ultimate. THEN the shockwave pulls malzahar, ending his ultimate.
In a replay of the bug, I would want to see exactly when orianna ultied, as when Malzahar did. I would also ask that you test this with Malzahar going straight from a silence into ulti; if ori can ulti him out of that, we certainly have a problem.
I am choosing to ignore if Ori had a QSS for some reason, or a support had crucible, or someone else interrupted Malzahar.