Every once and awhile I've seen pretty much all of my animals do this. I use a hairdryer to heat my thawed rats up just prior to feeding. I chalk it up to "maybe there's a spot warmer than the head somewhere on the rat". Typically, they end up finding the head on their own and finish swallowing without a problem. If I see that they're having troubles finding the head after searching for a bit, I'll pull it out and pretty much blast only the head of the rat with the hairdryer again for a few minutes until I'm sure it won't confuse them. I wouldn't be concerned about anything beyond that as long as they're still eating fine.