Looks like dominance issues to me. Has she ever been corrected for this behaviour?
The other thing that I would want to look at is what is going on right before she starts this. Also what is her daily life like? There are a lot of factors that can be playing into a behaviour. Frustration, lack of exercise, lack of free herd time etc. But from the video it looks like she's acting out with frustrated aggression. Is she in cycle?
Does she see you as lead horse? If she does then this would explain why she does not do it in your presence. To be honest it looks like she is "asking" to be dominated/corrected. Hence the challenging.
Just on this little bit of clip it looks like she is challenging the other horse and because of the fence both are unable to connect in the normal way horses would. Because the other horse is clearly unaffected by her dominance moves and does not move off when she tells him she escalates to the point where the clip starts.
Ok, resist the urge to laugh out loud...have you ever "claimed" the area for yourself, including the fence?
So yeah, no real answer but questions to consider because you have to look at the whole picture that got her there and not just the end result.