I'd go with the genset overheated and shutdown as a result, as the explanation. And, if there's any part of the fuel line that is metal, it may have created an air bubble in the line and consequently a vapor lock.
900ft elevation is not really all that much when considering air/fuel mixture. In aviation, small-plane pilots will start to lean out the mixture when they get up around 2K-3K feet to compensate for the thinner air. If 900ft elevation actually did affect the mixture, the genset would likely have run a little rich and maybe smoked some, but I doubt it would have shut down, and if it's running rich, the extra fuel would cause a cooling effect on the engine.