Tesla wanted to show off all the best features that the Cybertruck had to offer including the shatterproof glass on the windows. To demonstrate that, they decided to toss some metal balls onto the glass which should show that the glass will not shatter but it did.
After getting made fun of for not trying it out before officially testing it in public, Elon Musk released a video on Twitter showing them testing it out behind the scene and sure enough, it worked.
Musk also explained that the reason why it broke because before the test, they tested it out with a sledgehammer on stage and that have already cracked the base of the glass and lead to the window shattering when they threw a metal ball later on.
We will have to see if the glass will really stay intact when it officially arrive in 2021.