The figurative expression used above, which is known as personification, is mostly used by authors and poets to give inanimate object life or feeling that is only possible in a lively being.
Just like a human can crouch or bend their knees in a squatting position, the expression above tries to paint same lively picture of the chair taking the same form in description.
This would be an example of a mood swing or emotional swings
Explanation: From my understanding, it is common for people with BPD to have intense swings of emotion(it's one of the most common symptoms) Marie's mood swings would be very intense assuming she hasnt began treatment yet.