If it is an attack and is violent then it is a totally different situation from a hen mounting a hen. When a dominant hen is mounting another hen she follows much the same sequence and actions as a rooster mounting a hen. I'll copy something I wrote a few years back about roosters and hens for...
To me a dominant hen mounting another in the absence of a rooster is not dangerous abhorrent behavior. It is not something I see that often but I have seen it. It is simply her showing her dominance. It's a way to enforce her authority as the dominant hen. As long as no one is being injured...