Why do pandas have a high probability of giving birth to twins?
Charnov and Ernest (2006) present data on offspring number per year and neonatal mass for 532 species of mammals. The two are related by the linear regression equation:

ln(offspring/year) = 2.4 - ( 0.3 * ln(neonate mass) )

Giant panda neonates weigh 100-200 g and are weaned at 46 weeks. 

So, according to the regression, pandas should have, on average, 2.8 to 2.2 offspring per year (for 100 and 200 g respectively). With a weaning time of 46 weeks, they could have 1.13 (52/46) litters per year. If every litter were exactly twins, that would be 1.13 * 2 = 2.26 offspring per year, which is within the predicted range.

Humans (neonatal mass of 3400 g) are predicted to have 0.96 offspring/year.

Charnov EL and SKM Ernest. 2006. The Offspring‐Size/Clutch‐Size Trade‐Off in Mammals. American Naturalist 167:578-582.