M3 LR severe fishtailing under heavy braking

[init6]

Every day is a school day
A delve into Wikipedia gives this article too:

Fishtailing - Wikipedia

en.wikipedia.org

"Similar behavior is evident during heavy braking in all types of road vehicles, due to weight transfer to the front. This can be mitigated by re-proportioning the braking forces (more to the front, less to the rear) to keep the rear wheels from locking up. Most modern cars use anti-lock brakes (ABS) which addresses this problem."

So the question for the OP is whether or not the ABS or traction control (whatever Tesla are using to correct fishtailing) is behaving as expected. Given Dilly's experience above and the surprise of the CarWow driver, it MAY not be.

There's a phrase commonly used by Tesla that may come up in discussions with them though

 

[Tevo Solutions]

A very small amount of lateral movement can feel like a lot if you're not used to it happening and most drivers don't get the chance to practise emergency braking until they find themselves in an emergency.

These cars don't come from the factory with the brakes bedded so it's possible there was some inconsistency in the effectiveness of the brakes from one side to the other when you stepped on them hard.

There are other possible causes but without seeing and testing the car it's impossible to say whether it's a problem or just normal for the conditions and state of the car at the time.

I saw a Model 3 last week where one of the rear suspension arm bolts had loosened and ended up falling out. That would certainly cause some instability under heavy braking!

OP: Is your LR completely as it came from the factory and what cold tyre pressures are you using?

We shot this video a few weeks ago and carried out probably 40+ emergency stops from 60mph during the day without any unusual behaviour at all.