The word alight has two distinct meanings: it can mean coming down or settling in a delicate manner, such as a bird perching, or it can be a rather poetic way to describe something that’s on fire (or “afire”).
Just as the word alight has two distinct grammatical forms and meanings, it has two distinct beginnings in the Old English. That period’s word ālīhtan had an original meaning of dismounting, or lightening the load on the horse, and so is the ancestor of the verb we use today that means "to settle or perch." Meanwhile, the word onlīhtan (see the different spelling?) of that same time meant to be on fire, which became our adjectival use of the word.