What causes mold to grow on frozen meats?
Frozen food does not necessarily stay edible forever. There are some microbes that love (even extremely) cold climate. Such microbes are called cryophiles and can survive or even need temperatures as low as -15°C. There are microbes that show activity at temperatures as low as ~-200°C (http://microbewiki.kenyon.edu/index.php/Colwellia_psychrerythraea).

However, if you store your food at at least -18°C, most microbes will go dormant and you can store it for a long time without problems. Above that, however, there are several microbes that can and will grow and spoil your food, though I can't tell you which ones in particular. That's the reason frozen food bought in a store (in germany at least) has a label how cold it has to be stored for the minimum shelf-life to apply (often includes a table for minimum shelf-life at various temperatures as well).