It could have been so many things! The problem with snakes is you hardly ever know when they are sick. They cannot limp or whine like a dog when in pain. The do not make facial expressions and the only sound they make is a hiss when upset. For all you know he could have had kidney or heart failure. Perhaps a collapsed lung or a digestive tract parasite (which would explain the regurgitation).

Out of all my snakes, right now one could be sick and I would not even know it... that's a sad thought but something us snake keepers have to be prepared for, even if you only have one.

You could get a necropsy done, especially if your concerned about it being viral and passing on to your other snakes but I really doubt this is going to be a problem.

Sorry to hear about your ball python. I wouldn't give up hope on your other snakes though. Good luck.