One or two through-faults? Probably fine. Twenty through-faults? The cumulative mechanical fatigue can loosen windings, crack insulation, and eventually lead to an internal catastrophic failure.

Under normal operation, the load impedance limits the current. During a fault, that impedance drops to almost zero. The only thing limiting the current is the transformer's own internal impedance—primarily its leakage reactance.

Imagine a dam bursting. Millions of gallons of water, once contained, suddenly surge through a tiny crack. That is, in essence, what happens inside a power transformer during a short circuit fault.