Mrs Danvers is sick of Mrs de Winter and she wants to kill her, so she tries to make her suicide herself to not be the guilty girl after the murder.
Yeah, just like this...
Mrs de Winter doesn't understand at all what's happening. She knows she's not really liked by Mrs Danvers but she still thinks she doesn't deserves her meanness.
No, YOU come with me
NOOOOO ! H-HELP !
Mrs Danvers then decides to appease her victim to make Mrs de Winter trust her and approach the window.
Mrs de Winter sits at the edge of the window. That's what Mrs Danvers wanted so she put her hand on Mrs de Winter's back and tries to pull her down the floor, at the bottom of the house.
For revenge and because it is absolutely not fair Mrs de Winter take the shoulders of Mrs Danvers to make her fall with her, now it is to late to get safe.