Astrowoman said:
Aha, I know the answer to this one! If the battery is in danger of overcharging, the ICE will come on to bleed voltage out of the system. Should only happen if the battery is totally green and you are still regen braking.
However, it does seem there ought to be a way of simply not continuing to charge, oughtn't there?
Well, there are at least two other ways, but both have drawbacks.
1. Use the hydraulic brakes to slow the car. Since it's "brake-by-wire", this can be done even though the driver does not press the brake pedal. But it adds additional complication to a life critical system and engineers hate to do that.
2. Discontinue simulated engine braking. Automotive engineers try to avoid unexpected changes in vehicle behavior as a confused driver can be a dangerous driver.
Also, there's at least one other reason why the engine might have come on, namely to warm it up. The (relatively) cool air rushing past the engine and catalytic converter may have cooled them to near the bottom of the operating temperature range or the cabin heater may have removed too much heat from the engine coolant.