Why do my hamsters fight?!?

my hamsters were bought together, and were together for a short time, Bambam always sat on Pebbles, i just thought, right, Bam's the boss. Then,i kept hearing squeaking, i looked in their cage, and they were rolling around squeaking. Pebbles was affected more on the fights, her ear was all split and she had a scab on her back, she still does now, they are both separated now, but when they look at each other, they look like magnets repelling, the Pebbles starts squeaking like mad and falls back and darts back when you want to pick her up to comfort her.