why does lily compare her parents to "birds trapped inside a closedroom"

Yes, I remember this quote. Her parent's anger and rage seem to have no direction. They scream at each other not fully comprehending what they are trying to say to each other. The energy is chaotic and heartbreaking,

“I don’t remember what they said, only the fury of their words, how the air turned raw and full of welts. Later it would remind me of birds trapped inside a closed room, flinging themselves against the windows and the walls, against each other. I inched backward, deeper into the closet, feeling my fingers in my mouth, the taste of shoes, of feet.”