First off, no; buszokotra is never, ever, ever, never in a million years correct, for a similar reason that it's never, ever, ever correct to say "the a buses". In buszokotra, you're trying to stack two different cases on the noun. In "the a buses", you're trying to stack two articles next to the noun.
Okay, so, as far as terminology: there's no such thing as the "subjective case". It simply doesn't exist. If you mean the case that marks the subject (i.e. the default form that nouns come in), that would be called the nominative case. The nominative of busz is, well, busz.
The nominative case doesn't "define an object" - as previously mentioned, it marks the subject. To mark an object, it depends. "Object" is an umbrella term for different grammatical categories: direct objects (marked with accusative case), and indirect objects (marked with the dative case). The accusative of busz is buszot; the dative of busz is busznak.
The difference between direct and indirect objects is notoriously difficult to explain, but let me try: the direct object (ACC) is the thing being verbed. When you throw a pen, the direct object is the thing being thrown - namely, the pen. When you chop a tree, the direct object is the thing being chopped - namely, the tree. When you play a game, the direct object is the thing being played - namely, the game.
In fine, the direct object receives the action.
The indirect object (DAT) is the thing to which the direct object is verbed. It's indirect, because it receives the action by way of an intermediary, the direct object. When you send a letter to John, the thing being sent is the letter, and the thing to which the [letter] is sent is John - he receives the "sending" by way of the letter, if that makes sense. If you throw a ball to your friend, the thing being thrown is the ball (ACC), and the thing to which the [ball] is thrown is your friend (DAT).
The direct object receives the action, and the indirect object receives the direct object.
(Just to muddy the waters: where there's an indirect object, the intermediary is not always directly stated. For example, in the sentence Susie talked to Bob, Bob is still the indirect object, even though the thing carrying the action of talking from Susie to Bob isn't mentioned. Implicitly, it would be something like "words", but it sounds odd to say Susie talked words to Bob - not least because "to talk" is what's referred to as an intransitive verb - i.e., it's forbidden to have a direct object (or an explicitly stated one, at least).)
So the problem with buszokotra, which has both the accusative and sublative cases (which is a no-no in and of itself; one case per noun, always, even if that case is the nominative which is completely unmarked), is that it shouldn't have the accusative. The buses are not the things being fallen. Again, all the more so because "to fall" is intransitive - it can't have a direct object, and thus nothing linked to that verb should be marked with the accusative.