I think I found the cause of this, on this web-site: http://www.weightlessdog.com/ct90.nsf

"Can this run without a battery?
Yes, a CT90 or CT110 can be operated without a battery. However, you don't want to do this for very long. The reason is, the CT series does not have a conventional voltage regulator. The battery performs most of this function. If you have no battery in place (or the battery is badly shorted) when you goose the bike's engine, the voltage will spike, too, generally blowing out every light bulb that happens to be on at the time, and occasionally trashing the rectifier."

When I put my new light in, I had forgotten to hook up the battery. I think it did have a low-beam when installed, but not after I buzzed around the farm with it. I won't do that again.

I hate to reveal my mistake, but maybe it will save someone else a few dollars.