I gave it a go:
The blue dot in the middle is the CoG. In normal ops, the green line is the axis where the MLG supports the aircraft. This axis is AFT of the CoG, hence no problem at all.
In the second case, the right MLG is not extended. So the aircraft will sit on the left MLG and the engine pod. The orange line illustrates this. It is the axis through which the aircraft is supported now. This axis is forward of the CoG, so the airplane will start tipping backwards.
Bear in mind, this is a rough sketch. If I have time tonight, I might make a more detailed version, with actaul Airbus drawings. For now, I hope this explains what might have happened in my eyes.