E used to do this quite often. I took it to mean that he was still hungry and tried giving him more, but it got to a point where I was over-feeding him. Babies can get milk much faster through a nipple, even the slow flow ones, so like Ree723 said their brains don't always have time to sync up. Kellymom has some good tips to combat this http://kellymom.com/bf/pumpingmoms/feeding-tools/bottle-feeding/
It was ultimately just a phase for us and E will eat without crying afterwards now.
