Very likely since now everyone knows about Tesla's new truck and I doubt anyone in the market for such a thing really cares about indestructible side windows?
It's slapstick comedy applied to PR.
It's either clever marketing or a happy accident.
SpaceX failed launches are marketing too, but they're not stunts. They are live tests that are filmed anyway. The fact they share the video is secondary to the main goal of the tests.
But this? It's a staged marketing event. It's not a test procedure. The point is to get as many people to see it as possible. Smashing the windows doesn't cost much or stop a potential buyer in their tracks.
The windows break easily in a highly visible but safe way (no broken glass fragments spilling onto the stage, no projectiles bouncing back into the audience). They repeat the “demonstration” with the same “accidental” result instead of just moving on. Musk knows it's fun to watch things go wrong, and it boosts the already high newsworthiness of the event.
It would have been fun to watch anyway because company founders don't tend to let people throw things at their latest vehicle on stage, but that alone wouldn't make as many headlines as an accident.
"Armor glass" that completely shatters when you throw something at it by hand? Come on...
Also, why would it need armor glass in the first place? They picked a gimmick that is irrelevant to the product so that this "failure" does not hurt at all.
I mean, as long as we're rejecting Hanlon's razor, we might as well consider that someone could exploit the way TSLA fluctuates wildly every time Elon so much as tweets.
(No, I don't believe this.)
It invites people who don't care much for Tesla or the vehicle itself to share the video, it softens the seriousness and tension of a launch, and it helps to humanise the brand and the presenter.