I can think of a few possible but highly unlikely scenarios for a typical bolt action rifle to do this, but nothing that would be easy to duplicate. Generally the bolt handle will be at least passively 'locked' in place by the geometry of the bolt and rifle, and sometimes actively locked in place. If the bolt handle isn't fully seated, there really shouldn't be a way for the firing pin to release. Wear and lack of maintenance could cause an issue like this, but I would expect the incident would be repeatable.