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Because like all rockets it was designed to handle any expected aerodynamic loads with a certain margin with minimal mass.

Essentially the right parts just popped like a balloon. Falling on your side involves much different loads than being propelled into orbit. There's little value to designing a rocket to withstand such a fall.

I assume it's due to the liquid oxygen remaining in the booster; essentially its a towering pressurised container falling on its side causing a breach.

Different stresses.

To elaborate, the rocket can take only about 1 atmosphere of pressure in the radial direction, but much more force in the longitudinal direction. It's designed to withstand the stresses of going in a straight line, but as soon as it bends it'll crumple, like a toilet paper cardboard tube.

So when the thing topples over, it bends. And when it bends, it ruptures because it's far in excess of the loads under standard conditions.