For every action there is an equal and opposite reaction – Newton’s 3rd law of motion.
If Zenyatta kicks someone hard enough to knock them back, he would fly back as well, since he doesn’t have a foot planted into the ground. It would be like Ashe’s coach gun.
It’s a video game of course, but with Zen floating it does feel a bit wrong without any reaction. If the kick makes it into the game as is, the animation team will need to put his feet on the ground somehow. 
He’s using his spiritual monk powers to hold himself in place
He’s already bypassing the force of gravity, so it can be assumed he’s using the same power to prevent his own knockback.
All his kicks are flying kicks 
The law isn’t that he would be knocked back, it’s that an equal force would be applied to him. We don’t know how he moves and floats so it’s reasonable to assume the force keeping him in place is greater than the force from the kick.
Also it’s a video game lol
He might also be a very heavy robot
He is in fact so heavy, that he catches enemy Tracers, Sombras and Genjis in his orbit at a regular base 
Physics were patched out ages ago. See Roadhog ult knocking people back at a consistent rate regardless of range.
Zen’s mastery of his body is such that he can float above the ground at all times in defiance of gravity. You really think it would be a challenge for him to keep himself from being thrown by his own kick?
“For every action there is an equal and opposite reaction, until Zenyatta shoves his foot up your backside” - Sir Isaac Newton.
Whenever the laws of physics are broken there’s one answer
MAGIC
Sorry. I made a similar thread without reading yours.
His kick should not boop, just throw him backwards
Ah yes. “Spiritual monk powers”. We have dismissed that claim.
I don’t think Newton accounted for Omnics when he noted the law of motion. For all we know, Zenyatta is basically standing. There could be invisible robo BS that’s tethering him to the ground.
Or, you know, he could have a magnet on his butt or something that like, suctions toward the earth’s core.
MAGNET BUTT.
BUTT MAGNET.
I say we go with that.
How do you think Zenyatta floats?
With rockets. He isn’t being knocked back because the rockets are cancelling out that force.
Oh? You don’t hear or see rockets? Well, if Blizzard put them in, they’d be deafening and so they were omitted for gameplay reasons.
Maybe he should learn how to do that when people hit him sound waves…
It’s safe to say zenyatta never skipped leg day
So what you’re saying is… We need to let Zenyatta superjump by kicking the ground?