If it was an issue with the control device, it would tend to be erratic and unpredictable as to what specifically happened. If there was any way to make it act reliably, that would involve changing something with the control device(touching one of the affected buttons, etc...). And of course changing the control device would change it as well.
If it happens with all controllers, in a very smooth, steady way, and always in the same exact manner, and always in the same cars every time they are used and never in any other cars... it's the car. This is further shown by the fact that the car is what controls the effect -- as the car rolls and decelerates with no input, the same thing happens at the same RPM every time, and changing gears without touching any other button changes the the throttle "input" in a predictable and repeatable way.
The OP's video may not be the best example, but having played around with this a little after stumbling upon it myself I am certain the video is showing the programmed behavior of the car, not any control issue. It's just the way those certain cars work, as a few others have said.
I admit I was slightly surprised that people who have been active on the game and on these forums for a long time would have not been aware of this by now(particularly considering how popular the GT-R is), but if you haven't read the right posts or looked at the HUD telemetry while it's happening or while rolling down to a slow speed in high gear in the right car then you could easily not notice. It's not something that is likely to ever present itself in a racing situation.
. It doesn't matter if you use a wheel or not, - it'll still do it, until you use the handbrake.