I think it's due to the weight distribution. The MacBook Air is bulkier at the back but extremely slim at the front. If you position your hand closer to the back, the front being lighter will mean that it'll not topple over.
I think the MacBook Pro has a more even distribution of weight around the base, so the front of the laptop's base is heavier, making it easier to topple over. It just takes more effort to balance it one handed.