Nasty - that eruption is typical for Damascus. Nothing too violent, just blown out the top - rarely creating a problem for the shooter. It appears to be an obstruction blow-out - perhaps double loaded on that side or perhaps a space developed between wads, shot and powder due to the hydraulic effect of sealing wads which pushed the load or part of the load back up an inch or so off the powder. I have also seen that blow caused by shoving the tubes into the water to shoot a fish (story given) and pulling a trigger. The gun in question was a Westely Richards double with Damascus barrels. W.W.Greener's 9th Edition of "The Gun and it's Development" also has just such a picture- a double with the tops of both breeches blown out.

It can happen - one must know what one is doing.
Unfortunate it happened to what appears to be a nice old German, Swiss or perhaps Austrian gun. (just a guess based on the wooden trigger guard and lock and/or makers name.

The 'break' must have happened a long time \before the gun's use was stopped. Note the 'extra' wear and patina around the nipple seat on the right barrel, showing more use of corrosive caps. That is just speculation, as there could be a number of reasons for what seems to be more 'wear' on the right side, ie: method of storage.