My DD1 used to do it all the time especially with meat. She would chew, chew, chew....then spit it out. Would swallow sausages and mince and spit out other meats. She would also do it some other food but my thinking was she got out most of the nutrition just not the fibre which she got other ways. A bit gross at the table but we learnt to live with it. It took quite some time for her to swallow the foods she couldn't but she is now fine and can now eat everything with no problems.
Someone told me it had something to do with the gag reflux..not sure how true this is.