> descended from the trio section of marches

.... and in turn from the minuet and trio, which became "the standard third movement in the four-movement classical symphony".

But why eight bars? My mind can't rid itself of the picture of a tunesmith in Tin Pan Alley who's paid by the bar ("Whaddya mean, giving me a tune that only fills half a page?"), who happens to have one-and-a-half tunes in his head, and zips them together to save wasting the half-tune. Guess how many bars long it was :-) ?