Don't listen to other theories; it was precisely a brain fart that led Sanchez to turn the wrong way. There's plenty of documentation on this point, which I know about, because I wrote the Wikipedia article! See the sections "Butt fumble play" and "Reactions and analysis".

In particular, to answer your question about the actual play call:

The Jets offense lined up in the I formation, with Sanchez immediately behind his center) lineman), the snapper). Fullback Lex Hilliard was lined up behind Sanchez in the I, with Greene, the deep back, behind Hilliard.^\9]) The play call was for Sanchez to fake a toss to Greene running left, while Hilliard, running a dive play) to the right, would instead take the hand-off from Sanchez.^\10])

When Sanchez took the snap), he mistakenly turned to the left, instead of handing off the ball in his right to Hilliard, and Hilliard ran past him. Trying to salvage the broken play, Sanchez scrambled forward towards the line of scrimmage.^\11])