That is a very spectacular burst, but I would be surprised if there wasn't some cause other than being for black powder.
The gun is Belgian, and "PV", for "poudre vive" means it was proved for nitro powder. The crown on the oval "ELG" means it was proved in 1893 or later, at which time the use of smokeless powder in a .303 would have been taken for granted.
Even in British manufacture, although nitro proof was only introduced in 1904, no .303 barrel was explicitly intended for black powder. They knew before the adoption of the Lee-Metford in 1888 that the French had a "chemical powder" which gave enhanced power, and the only question is whether they knew it also gave enhanced pressure. I think they did, for the sights f the Lee-Metford were graduated from the start for a flatter trajectory than could be obtained with the stopgap solid black powder pellet they used.
In fact the secret of the Lebel were pretty well out, at a time when French soldiers were threatened with ten years in jail for opening a cartridge - and being French, probably all did. There is a most interesting story which may even be true. In 1887 one Schnabele, who sounds like an Alsatian, deserted to Germany with his rifle and ammunition, and offered the to the War Ministry for 20,000 marks. They told him to go away and not be so silly, as the rifle was little more than a smallbore Kropatschek. So he knocked on Bismark's door, and the old man brought the Ministry into line. This illustration was published by Professor Hebler in Leipzig in 1890, and bears the British Intelligence Division's stamp for 1891. I don't have the book, and am not likely to get it, but the pressures of smokeless in smallbore military rifles were surely well known, and black powder cartridges made for them entirely out of use, when this Martini was made.
Many thousands of rounds of MkVII ammunition have been fired in Long Lee-Enfields, without mishaps that I have heard of. If it comes to that, unless the burst Martini was a wallhanger from new, it must surely have fired a lot of cartridges with higher pressure than 12gr.of Unique. When the British proof houses decided to prove the No4 but not the SMLE for 7.62x51, it was due to cracks developing in the lighter SMLE receiver, not failures of the barrel.
Belgian proof was pretty reliable, and Martinis were one of the firearms they did best. I suppose it is possible that a barrel with seams or inclusions might pass proof and let go later, but I don't see any part of the fracture which doesn't look like new-riven metal. My guess is that a doiuble charge of Unique is the most likely cause, and second is a "misfire" ejected without noticing that a primer with no powder has left a bullet lodged just ahead of the chamber.