How can mold grow on food in the freezer?

The same thing has happened to me, twice, but only with the same brand of tortillas. (Trader Joes, I believe). It has also happened with their sourdough bread. I have bought these items fresh and put them in the freezer anywhere from right away, to within 24 hours. My freezer was working properly without any malfunctions. I am also wondering what would cause it?

I'd say your freezer doesn't work properly. To check, put in ice cubes in a bag / box, and check them. Once they aren't cubes anymore your freezer (and its contents) thawed.

Molds won't grow below freezing but they can easily survive the process. With that in mind, possibilities include that the mold grew before you put the tortillas in the freezer, while they were cooling in the freezer, and after they were removed from the freezer. Mold could also have grown during repeated defrost cycles or because your freezer isn't working properly.

There are some bacteria/ molds that grow at certain temperatures. The one that causes the most food spoilage occurs at the medium range of 68-113 Farenheit range. These bacteria are known as mesophiles. They have molds and bacteria that grows at even hotter temperatures or in your case, colder (psychophiles aka cold-lovers). Either that, or it is just a collection of unknown materia (could be from packaging dye scrapings) that collected on the ice. Hope that helps.