It’s the Moro reflex which makes her get startled easily. This is a natural reflex that usually goes away by about 6 months old. Swaddles may help, but if your baby can go back to sleep on her own, then you can leave her alone:) teaches her to self soothe too
May be having some nightmares .. tap him n put some bed Sheetrock pillow on back of hiM it may makes him secure ..