In the first weeks of lactation, your "letdown reflex" the reflex that releases the milk produced by your breasts is still being tailored to respond to the sensation of your baby nursing. Frequent nursing will help the reflex become attuned to that particular stimulation, but until then, many sensations and even thoughts may trigger it.
Hearing your baby or another baby crying, thinking about your baby, smelling your baby, or sitting in the chair where you usually nurse your baby might be enough in these early weeks to make your body to release the hormone oxytocin. Oxytocin causes the cells in the milk-making lobes of your breasts to contract and eject the milk toward the nipple, where your baby can pump it out. (If your baby isn't nursing at that moment, the milk may leak or even spray.)
The more often you nurse in the early days and weeks, the more quickly your reflex will become attuned to your baby, and spontaneous letdowns (and leaks) will gradually disappear. (There's a wide variation in normal breastfeeding experience, however, and many women never leak.)]
