it has often puzzled me, on the occasions when such things come to mind, why "trousers" should be a plural noun in the first place.

after all, nobody with the possible exception of Long John Silver has ever owned "a trouser" , have they ?

the usage has also , curiously, emigrated across the pond, where men wear "pants" instead of trousers, but still in the plural.

I would hazard a guess that this usage is a unique quirk of linguistics, but am as always prepared to be proved wrong.