Usually women work out a schedule for feeding. The body becomes used to that schedule. When it's time to feed, the the breasts are full. At that time, if they are stimulated in some way, whether physically (brush against something), visually (see a baby) or emotionally (even thinking about your baby) can cause them to leak. Suppose a woman is somewhere where there is no private area. It can be embarrassing to have two big wet spots appear on your top. If you go long enough, it can become painful.
And men who can't get over catching a glimps of a woman's breast while breast feeding are perverts. The experience, for a woman, is so beyond the sexual that it borders on ridiculous. It's a bonding experience beyond compare.