Rich,
That's a pretty small generator and outputs only 20 amps at 120 volts. Even though the sander doesn't draw any where near that load, it is very likely that the generator's output frequency (engine rpm) is fluctuating when the sander begins to draw power. I spoke with my contact in the service department about this topic recently, and even though we don't have a definitive answer, we suspect the electronic controls are shutting down the tool because it gets confused when it sees significant swings in the frequency. Line power (mains power for you European folks) will have very steady frequency, so the sander is expecting a constant frequency.
This is just a shot in the dark, but if you have a dust extractor, try turning this on to manual before starting the sander (plug the sander directly into the generator). The reasoning behind this is that if the generator is already partially loaded, the slight increase in load from the sander will not be quite as drastic. The generator's governor may be able to maintain engine speed better.
