So the masks give them their powers? I'm guessing the masks fuse to their face once applied like Jim Carrey's "The Mask" which would explain why their eyebrows then appear on the outside of the mask (note in the concept art that the eyebrows are not on the masks when detached).
Guessing they woke up one day with the masks on their faces, and the masks presumably came from... ALIENS!
Guessing they woke up one day with the masks on their faces, and the masks presumably came from... ALIENS!