Genetic Mozart
Without actual experimentation it is impossible to know for sure, but I would be very surprised if the ability to write minuets at the age of five, and symphonies at the age of nine, did not have at least some biological or neurological quirk attached to them.
Otherwise, one would think it would be possible to routinely school children to be able to do this.
An important data point would be to find out which educators taught Mozart and his sister, and whether or not such prodigies were unique or not among their students.