Some mothers have such a strong let-down that the baby cannot handle the volume of milk.
If your baby chokes, gags, or pushes off of the breast a minute or two after beginning to feed, an overactive let-down may be the cause.
Most babies do learn to handle let-down as they mature, but until then you might take the baby off the breast until the milk flow slows. Try using only one breast each feeding. Some mothers find it helps to position the baby so that the back of the baby's throat is higher than the nipple, so that the milk has to "travel" uphill during a let-down, which slows the flow. Another option is to try pumping through the let-down immediately before a feeding.