The rocket was undergoing a static fire test of the stage, in which a vehicle is clamped to a test stand while its engines are ignited, when the booster broke free. According to a statement from the company, the rocket was not sufficiently clamped down and blasted off from the test stand “due to a structural failure.”
Video of the accidental ascent showed the rocket rising several hundred meters into the sky before it crashed explosively into a mountain 1.5 km away from the test site.
That’s for testing engines alone. Static fire is separate yes way further down the line when you have the rocket built and ready to fly