He had no leg to stand on and so what could he do but fall down, which is exactly what he did. What else can a man with no leg to stand on do but fall down? Well, there may be plenty he can do, but still, in this context... But, you will be wondering, if he had no leg to stand on and he went and fell down, how did he in the first place get up to this height from which he fell down? And why, knowing as he must have done, that he could not but fall down once he got up there, did he go about satisfying the how as to the getting up there?
Ah but, you might retort, the how is easy: once it's not impossible it must be merely a matter of methodology, and so all it requires, apart from its being impossibile, is the stumbling, through trial and error, intuition, or basic knowledge of some kind, onto the wheels of this methodology and following obediently in its wake to the logical destination.
So that's the how, at least in general terms, but the why. Why would he go about, whatever the methodology, getting himself up to a height from which he could only fall down? But isn't this presuming a bit much. He mightn't have considered he'd have to fall at all. He might have suspected it, even assumed it to be most likely, but still he might have possessed some sliver of hope that this falling would not have to be necessarily so.
But on the other hand he might not have cared one way or the other as to whether he'd fall down or not. It might have been purely in the way of an impartial observer that he looked on all this, simply as a matter of curiosity: would he or wouldn't he fall down. He might even have kept on getting himself back up there, even after falling, to establish precise laws of probability as to the falling or not falling.
And then again for all we know he may have even wanted to fall down. Some perversion? Perhaps but perhaps just again in the way of being scientific: the falling down may have accorded with his expectations, calculations even; and with the falling, as he's hurtling or just stumbling downwards - it may after all have just been a most humble height - he's pleased with the falling.