With respect. If the action was locked at the point of fireing then it is impossible for the rifle to open from excessive pressure without substancial structural damage to the action and lumps.

It is far more likley that the action was only held by the tip of the bolt either because of foreign matter or rim thickness or just simple stiffness being new and unfamilier. On firing it opened, this alone would allow the case to move suficiently to streach and seperate. An unsuported case, even a low pressure one, will seperate, the brass will not stand it if not suported by a chamber.
As a matter of habit I thumb the lever to the left to check bolting if time permits on loading and periodicaly on a stalk. But then I am a two paper bag man.

Quote The fact that there was case separation confirms that pressures were too high for what ever reason - high powder charge, tight throat, hard crimp, etc.....Quote

As above, that is not necessaraly so.

Regards