It used to be (through the mid 50's?) what was needed in your area and 3/4 of the people lived in the Philadelphia "area". My guess is that it is now whichever mint has the most spare time. BTW, you do realize that they (mostly Philadelphia) probably make most of the rest of the world's coinage (aside from Europe).
I prefer to think the directors from both mints play bloody knuckles (stupid high school quarter game most males play) to determine whose mint will have the responsibility to mint the most.
The US mint has not made coins or planchets for other countries since 1984 with the exception of the 1000 Kr coins for Iceland that were issued in the Millennium Set. Production is probably determined by the federal reserve based on historical requests from various areas of the country weighed against what is held on hand at the various federal reserve banks and branch banks. Over time they they know about how many coins will be needed each year, and they know what areas tend to constantly order coins and which types, and which ares are always sending coins back
