He didn't swallow it the wrong way, and he didn't choke. He swallowed it the right way, but the pretzel piece was too big and it got stuck in the correct passage. According to the doctor I saw explaining this, the pretzel (painfully stuck in the correct passage) stimulated the vagus nerve which slowed W's heart rate and rendered him unconscious.

Now here's the part that I, personally find very suspicious (and a little disturbing)...

On the very same day that Mr. B. got that pretzel stuck in his throat, perhaps even at the same time, I got a big honking vitamin pill stuck in my throat. I didn't pass out, but it hurt like hell. Could this be some kind of conspiracy? Hmmm....