I always figured it was your lower esophageal sphincter (the one by your stomach) kinda, like..spasming when you swallow, and then being seized up when the water tries to pass through, leading you to have to swallow extra-hard and force it through.

The resulting pain would be from having forced it, and feel like it's in your chest 'cause abdominal pains can tend to be pretty imprecise.

But, I know absolutely nothing about medicine or the human body, so don't take my word for that. These are just the ramblings of a filth-strewn peasant trying to make sense of the world around him.