Either he realized that, as fit as he can be, his neck at 40 isn't what used to be at 30 or simply that he wouldn't be able to keep up with the younger chaps and quickly backpedalled.
Anyway I saw some still unpublished pics of the Schumacher crash at Jerez (a local photographer was there taking promotional pictures for some teams and captured most of the event) and to call it "massive" would be an understatement.