There is also a clutch overheat safety procedure Mike, so it could have been that, and also sometimes the system will look for 1st gear, miss it (for whatever reason) and then engage 2nd instead. The manual doesn't list this as a fault, but rather the system following the CC logic.
If it was an isolated incident, then I'd keep an eye on it in case it happens again, but not be overly concerned.
Welcome to the forum by the way
How long have you had your GTS? Enjoying it other than the reason for your posts?