M.R.
It's a texture thing. lol. They've already learned that their tongue can have a sense of touch and now they are finding out that the rest of the mouth does too (but that it feels totally different).
Most little kids go through this phase (2 of my 3 did). It amuses them to do so. Remind them that they need to swallow and they normally will.
As long as it's not bedtime immediately after, or they aren't about to go running around with half chewed food in their mouth (a choking hazard) there's no real reason why they can't sit for a few minutes with their "chipmunked" food. Just make sure they understand they can't get up until they have swallowed all of it.