Illegal pitch or no pitch.

[jmkupka]

I'll take a crack at this...

For the same reason the pitcher cannot retouch hands after separating. The runners base their timing on the pitcher's actions.

Granted, no matter what the pitchers do (legally) in their mechanics (and there are some strange variations), the runner can't break until release; however, we have to judge that whatever caused the pitcher to stumble was enough to screw up the runner's timing.