If the margins are high enough, then it's optimal to overproduce and then sell to everyone who will buy at a certain price point. If discounts will reduce your ability to sell the same brand in the future at that price point, it can be financially optimal to destroy it instead.
And what about competing clothes sellers willing to sell at lower margins and hence lower prices to customers?
The reason this works with clothing for some brands is that some utility of the clothing is in the signaling it provides to the humans wearing it, and the signaling comes from the scarcity of that type of clothing.
A clothes seller selling clothing that signals the wearer is able to afford a more scarce type of clothing is thus ruining their own products’ value proposition by decreasing the scarcity of the clothes.
That is why it might be better to destroy than see it on sale at TJ Maxx or Walmart or whatever cheap store.
Bottom line is clothes sellers are catering to what clothes buyers want. They are not stupid about how to operate their business.
And o11c’s comment about glass makers breaking windows is irrelevant since the glass maker does not own the window they are breaking, whereas a clothes seller owns the clothes they might be destroying.
I think it's just lazy corporate employees didn't know what else to do with large volumes of excess inventory.