Using the plural for a garment worn on the legs dates back to Old English. The OED says that breeches came from the Old English word
Old English bréc, plural of bróc: A garment covering the loins and thighs.
However, in Middle English, the Old English plural bréc became the Middle English breech, which was often treated as a singular. The OED gives citations:
1535 Bible (Coverdale) Jer. xiii. 1 Get the a lynnen breche, and gyrde it aboute thy loynes.
But when breech became singular, people started using breeches in the plural instead.
1591 Spenser Prosopopoia in Complaints His breeches were made after the new cut.
(Both of these usages probably started a century or more earlier than my citations, but I chose the most unambiguous citations from the OED — for many citations it's impossible to tell whether the word is plural or not.)
So it looks like using the plural for lower-body garments has been around since Old English. There was temporary change of usage in Middle English, when the strong plural started being replaced by the -es ending, and breech was reinterpreted as a singular, but it didn't last.
So you will have to look back to Old English or before to find the origin of this usage.
This usage probably goes back before the split into North Germanic and West Germanic (ca. 200 AD), as trousers is plural in most Germanic languages (although not Dutch, for some reason).
Possibly both the original plural usage and the original garment are too far back in time to determine their origin for certain. If we go back 5300 years, we reach the times of Ötzi the Iceman, whose mummy was discovered in the Italian Alps wearing trousers made of two separate leggings.
I don't know when Ötzi's trousers went out of fashion, and joined trousers became de rigueur — presumably it was somewhere between 3300 BC and 300 AD. But we know that the plurality of "breeches" has probably survived for 1800 years, during which the actual article of clothing was usually singular. So it's quite possible the earlier answer is correct; trousers are plural because when the word entered the proto-Germanic language, people wore two of them.