Well ... it happened three times in a row this morning, one right after the other. So I think the "each first launch" theory is wrong.
And I think I can make it happen whenever I want now or, conversely, not make it happen.
I have a dual monitor setup, Vivaldi launches in the secondary monitor. If I am impatient and move the mouse over into the secondary monitor before Vivaldi is fully launched, it will shut down.
This seems particularly true, if I launch Vivaldi immediately after a quick succession of boot and immediate login when system things are still busy loading and Vivaldi is competing for resources. Later in the day, when the system has been up for a while, the problem does not occur much, if at all.
The dual monitor thing may be a red herring, though. I suppose I could test things by having Vivaldi launch in the primary monitor or by removing the secondary monitor temporarily and being impatient with the mouse. But I don't have time for this at the moment.
Anyway, if I wait a nanosecond or two for Vivaldi to fully start before moving the mouse, the problem, if it can be called such (minor annoyance might be a better description), does not seem to happen.