Don't know the official reason but I would guess that the main issue would be as follows:
1) Fuel leak could be coming from anywhere but wings/engines are prime locations for a leak.
2) In landing run fuel leak will be trailing fuel behind as the plane lands
3) Thrust reversers could/would push fuel (leaked) and air forwards an into the engine intakes. This is likely to cause a flame out of the back and cover the whole airframe in a mixture of fuel and air.
4) Atomised fuel air mixture is explosive/fire risk. In general it's better to leave this behind you than spray it in front of the engines :-)
So all in all probably not a good idea - would guess this applies to all jets with TR and not just the 320.
Just my thoughts - actual reason could be more obscure.