Wow, never heard of anything like that before.
The only things I can think of are:
1. Maybe they were extremely hungry? *shrug*
2. Maybe they aren't guppies- Perhaps some other type of livebearer like a mosquito fish. Mosquito fish are supposed to peaceful too, but maybe because he wasn't their species they considered him a threat. Or maybe the store gave you minnows.
3. I've seen guppies behave like tetras- ie fin nipping. Maybe its tail was just so long it was irresistable.
Have they had time to adjust to their enviroment? Maybe they're stressed- that could contribute to erratic behavior.
It may seem like a pain, but perhaps if you planted your 10G that they're in heavily, you could just let their fry survive. An influx of young fish (male and female) might shock them out of the behavior. Did you buy all-female feeders on purpose? Normally breeders only sell males since you only need one male in a female tank to fertilise them all. If you didn't request all females it makes me think they could be minnows- they all resemble a colorless female guppy.