Упалось vs Сдулось

Question

Can anyone explain why "упалось" isn't a proper form whereas "сдулось" is?

 [+] Упало     [+] Сдуло

 [-] Упалось   [+] Сдулось

Answer 1

No reason; it's a possible verb that never acquired a meaning to become an actual verb. It's just how you don't say "think down" or "open off" in English — it's not that there's something inherently wrong with the combinations, they just don't mean anything. In the case of *упалось, that's not surprising considering упасть is intransitive. A verb that doesn't take a direct object wouldn't take a reflexive particle either.

Answer 2

Сдуть means to deflate[blow away (e.g. by the wind)] something or someone, so it's about performing an action aimed at some object which is different from the subject.

Сдуться ends with -ся suffix. In this case it means the subject and object are the same, literally "to deflate itself/on its own" (btw, nothing can be blown away on its own, so in this context it would sound weird to me), without any action from outside.

A few more examples:

мыть -> мыться (== мыть себя)

вешать -> вешаться (== вешать себя)

-ся can also implicitly indicate the passive voice:

варить (to boil) -> вариться (== to be boiled)

When something falls (упасть only indicates the fact of falling, with no reference to the reason), there is no external object or force that causes the action, so such form (упасться/упалось), which would show the "isolation" of the action, makes no sense.

Answer 3

You can (often) basically think of the Russian reflective ending -ся as meaning "self" (with the respective pronominal addition).

In this case (yes, I'm using examples from Vladimir Kovalenko's reply), the verb "сдуть" can mean "to deflate" or "to blow off" (as in off some place), so "сдулось" means "(it) deflated itself", which makes sense; or it can mean "(it) blew itself off", which is kind of weird, so that meaning doesn't really exist in Russian.
(Incidentally, my spellchecker doesn't think that word exists. The verb certainly does, but its neuter form in particular might be too uncommon, since most of the objects that can deflate are not neuter.)

Meanwhile, the verb "упасть" means "to fall", which is intransitive (one cannot fall anything), so "it fell itself" doesn't mean anything, and similarly the Russian form "упалось" does not mean anything either.

You can, however, say "свалилось", from the verb "свалить" (to push down). Really, "it pushed itself down" is also kind of weird, but it's as good as a metaphor for something falling down for no visible reason as any.
(Why does stuff randomly fall down from shelves and other such places anyway? But that's probably worth another StackExchange question by itself... I think I should go and ask it, actually. As soon as I figure out what section it should be in, anyway.)