This is explained in Episode 2 when there is an investigation...
It's a tradition for the Providence crew to bring a quarter into the White Room as a good luck ritual.
The quarter was given to a technician before they entered the capsule.
We believe he must have dropped it on to the transom, or it fell out of his pocket.
The quarter was then lodged onto a a spring loaded dust cover of one of the forward booster separation motors on the starboard SRB right here. When the BSMs ignite, these dust covers, they fling open due to the 20,000 pounds of thrust generated by the BSM.
When this happened, the quarter was flung with enough velocity to puncture the LOX tank of the first stage.
The impact provided an ignition source, enabling the aluminum skin of the LOX tank to be rapidly consumed by fire, exacerbated by the presence of pure oxygen