The list of weird wine theories is long & illustrious.
As you know, the corks are under tremendous pressure when put in the bottle and significantly constricted. That’s possible due to the elasticity of the cork when it goes in, and we always see the cork spring back to some extent when the bottle is finally opened. However, I think that elasticity is lost if the cork dries out to an extent, which would set the end of the cork permanently into the smaller diameter. This makes the cork less capable of reacting to higher pressures, say when the bottle is jostled around and the carbonation activates. The cork then starts to elevate and fails.
At least that’s what happened in my example. Given that the bottle was in great storage conditions after it was put upright in the storage facility, and wasn’t leaking beforehand, the upright storage caused the problem.
Yet, curiously, the justification for storing sparkling wine upright is that the moisture in the cork from lying on the side softens the cork, making it more likely to fail or allow oxygen in as a result of the pressure.
(that’s the theory - not saying I subscribe to it)