What can be causing my dogs to fight?

Roxy (4yr old female mini boxer) has been attacking Maggie (2yr old female boxer). Maggie had been attacked by a pitbull and about a week after that is when Roxy started attacking Maggie. They have been together since Maggie was 8 weeks old and Roxy always loved her. Now I'm just not sure whats going on. There's really no pattern to Roxy attacking Maggie. One day they will be fine and then the next thing I know Roxy attacks. One thing I did notice though is Roxy has been sniffing Maggie a lot lately and before she attacks she sniffs, does like a dominance stance, goes to mount Maggie and then attacks. I have not tried anything other than keeping them apart cause I'm not sure what else to do. Any advice would be greatly appreciated.