Why "de" and not "sobre"?
In the sentence "...Y cayó de cabeza" someone fell on his head. But 'de' means "of". So he fell 'of' his head? Is there any logic here or is this an irregularity?
4 Answers
Nikki did a good job of giving examples of this usage. The particular reason is that in this case the preposition "de" is used to express the manner in which something occurred. It answers the question in what manner/how did he fall?
He fell (in this manner) on his head - de cabeza
He fell (in this manner) on his knees - de rodillas
Cayó de pie, como los gatos - He landed on his feet
I guess it´s just one of those things that is what it is....
cayó de rodillas... He fell on his knees.
cayó de cabeza... He fell on his head.
Just like the phrase... DE rodillas te pido... on knees I´m asking, like you are on your knees pleading to someone for something..
You can also say...
cayó de su bicicleta... He fell from his bicycle.
de would be of, and sobre would mean about. so in this case, de would be closest. 
Estar de pie - To be on your feet (standing)
and as nikkis said,
Estar de rodillas - To be on your knees (kneeling)
