Why is Molly so angry?

From the text:

She’s too old for this- too old to wait around to be placed with another foster family. Too old to switch schools, move to a new town, submit herself to yet another foster parent’s whims. She is so white-hot furious, she can barely see. She stokes the fire of her hatred, feeding it tidbits about bigoted idiot Dina and spineless mushmouth Ralph, because she knows that just beyond the rage is a sorrow so enervating it could render her immobile. She needs to keep moving, flickering around the room. She needs to fill her bags and get the hell out of here.