After many arm chair mechanics diagnosing the problem, I'm not sure what caused it. Here are the ideas:
A) Timing. The bike is running too advanced, or the mag's lobe is making one plug fire at 38° BTDC and the other isn't. I'm ordering a mag timing light to check this. The mag was professionally rebuilt and I feel pretty confident about my settings, but who knows. It didn't act up until now.
B) Jetting. I'm running a 19 pilot jet, which is one step bigger than stock, and 200 mains. It's accepted that the Bonnies should be running 190 or 200 mains, so I feel I can't be that far off.
C) Air leak. The only air leak I can imagine might be the crossover tube in the intake manifolds. I don't have a clamp on them. Again, it's never been an issue, but something to check.
D) The rings. The theory is the rings were letting in too much oil creating a super-lubricated piston, making the bike work too hard. Climbing the hill made it work harder than it should, creating a super lean condition.
Whatever it is, it sucks. I will probably have to split the cases. But I'm looking at the silver-lining, too. This will give me an opportunity to try to fix some motor issues, change the gearing, and finally take the bike back down to frame to do a nice paint job.
The last picture I took before it all went down.
Waiting in a parking lot.