I have a new Toledo, so presume the same applies to the Altea:
The heater output increases when you go into reverse because the car decides to engage 'Recirc' blower mode automatically to prevent exhaust fumes being drawn into the cabin by the ventilation system. That's according to the SEAT handbook.
Ingenious overengineering, isn't it?
Just think you could have been killed by exhaust fumes by reversing older cars over the years !