Translation:One of the apples fell to the ground.
The way I think it through: It makes it clear that the apple 'fell itself' rather than dropping something else. Some reflexive verbs are like that, as afeitarse, to shave. If I say 'I shaved this morning" in English people may assume I shaved myself, but in Spanish I can add the 'me' to make it clear I shaved myself.
You are right! "A + el = al" (same as "de + el = del", by the way). The only exception is when the article (el) is part of a proper name (titles, places, nicknames...).
You're talking about calling a kid: Llamaré a el niño = Wrong
Llamaré al niño = Right
But if you are talking about someone whose nickname is "El niño", you don't merge "a" or "de" together with "el", since the article is part of the proper noun:
Llamaré a El niño / Es de El niño.
One way to remember is thinking that if you were talking about Ellen, you wouldn't write "es dellen", but "es de Ellen" (it's Ellen's), so you don't do it with the article either :]