What are the factors that decide on whether you die instantly or get knocked out in PUBG?

As far as I can recall, there has never been anything that instantly kills you when playing in squads or duos. I always go down if I have a teammate. Of course in solos, it's always an insta-kill. The only times you instantly die in duos/squads is if you get knocked out while swimming/in a boat because your character goes underwater and you cannot get revived while underwater.

According to the Wiki page for Down but Not Out, on your 7th knock down, you have 0 seconds to get revived, which would indicate an instant death.

• You can enter the DBNO state multiple times during a match, but the time allowed for you to be revived will reduce each subsequent time (90s > 33s > 20s > 14s > 11s > 9s > 0s).

I can't say I've ever played a match where I've been knocked 7 times. I most likely was killed way before that.

I suppose another way you can instantly die is if you are the last standing teammate on your team while the rest of your squad are all downed. If you take enough damage to go down, you will instead die instantly, as will the rest of your team since it is physically impossible to get revived when all of your teammates are down.

I can confirm: in PUBG there is nothing that can make you go from up to dead instantly, except the 7 DBNO or if nobody can revive you.

But there are exceptions: You cannot be downed while swimming. If you would be going DBNO while swimming, you immediately die.

Also you could get comboed. There is maybe .5 seconds of invincibility while going down, but if you get rammed by a car, bounce forward while the DBNO animation is going and then rammed again, you will then die.

In a squad game, as long as you have team mates that can heal you up again, you will always get knocked out first when your health reaches 0.

If it appears that you died instantly by a grenade, then that grenade most likely has simultaneously knocked out your (remaining) team mates as well.

There can be other instances where it can appear that you died immediately like being run over by a car. The moment you get knocked out the moving car might still catch onto your downed body and will damage you again thus killing you. Basically any large amount of damage that gets applied to you right after being downed can make it look like you died instantly.