Don't think it's the batery, seems to be lying very low down so could just be a box that all the connectors are running into.
 
Taken from the SEAT press pack:
"Since the 2.0 TDI engine takes up the entire engine bay, space requirements have made it necessary to install the battery in the boot, although this does not significantly reduce luggage storage space or roominess."

So it sounds as if it wasn't done for weight distribution reasons.