With a case failure, it doesn't always fail fully around the circumference. It will usually take the path of least resistance, which is usually where the extractor is.
The other things is, when referencing the 'bulging', you have to remember that while the case is 'fully supported', that's only partially true in a lot of auto loaders.
Manually operated guns will have less of a chamfer or radius leading into the chamber where as auto loaders need more of one to allow for reliable feeding. You actually end up with a section just ahead of the bolt face that, while it's technically part of the chamber, is not actually in contact with the case and not really supported.
I'm sure everyone knows someone with an enfield with excessive headspace, and when the case head separates, nothing much happens. Mainly because the bolt only unlocks when you decide to unlock it.
With gas guns, as soon as you get enough pressure down the gas tube......it's opening, regardless of what else is going on. In a case where you have a failure through the extractor, while it's venting high pressure gas into the receiver, it's still allowing pressure to build up in the barrel which will push the bullet out the end and ultimately allow gas down the port.
I have seen many case failures that look just like the OPs picture. They are almost always from gas-operated or blowback/delayed-blowback guns. Most case head separations I've seen from manually operated guns, the rear of the case usually just pops off when you extract it, but not much else.
Who knows, maybe it did fire OOB, but I'm highly skeptical. Most people use OOBD as a catch all. Usually when OOBDs happen, the guy behind the trigger is hurt and has scars to show from it.