Perhaps the original was coined by a monarch, as "right as reign", or something similar. More likely, I think, is that it came about as a happenstance of conversation in an
agrarian culture, and became commonplace because of the alliteration.

Most interesting to me was Quinlon's 'right as an adamant'
(thank you, Bingley, for the link), adamant being at the time a "lodestone or magnet". Nowadays, if we speak adamantly, it means we stick tightly to what we are saying!
Cool!