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Cheap bulbs don't stand up to vibration as well as the name brands (and I suspect they are cheap bulbs). When the filament breaks while the bulb is it, it often vaporizes, so what you see is the tungsten coating the inside of the glass.
As Art said, the converter and batteries are operating in parallel, but the ocnverter may have switched its voltage depending on the charging condition, so the lights got brighter. Or some other load on the system switched off and there was less voltage drop in the wiring. It doesn't take much of a voltage change for lights to get brighter, or dimmer.
I would suggest you find out what type of converter you have, and if it has a multistage charger. You want one that has bulk, absorption, and float stages at least, and most now have an equalization stage that helps keep the battery plates from sulfating (getting a coating that reduces the efficiency and eventually kills the battery).
If your converter doesn't have these stages, then it could easily overcharge your batteries. Some converters have the capability built in, and others have an accessory that will plug into it to make it a multistage charger. Worth knowing.
Bob
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