it's the entering unawares thing, that meaning of fall, Spenser uses it in an early poem, though it has a couple of earlier usages, I think King James Bible, for one. I think a lot of the English usage is related, but shades of meaning, like to fall in with someone, an idea of happenstance, chance, etc. 'it so befell them that...,' but I still think the piece for the literary reading could be about the difference between that state and 'Love'.