I've already ran a few boxes of this brand of ammo before with no incident.
(in the gun we are talking about,?)
I consider this important information. Did the fired cases from the other boxes have the same bad bulges? Do cases from different ammo brands have the same bulge?
If so, it may NOT be an "overcharged round".
Essentially, ever since Glock proved they could get away with it, several recent pistol designs have chambers that are "generously" relieved in the feed ramp area. This does promote reliable feeding.
HOWEVER, the case is left with less support in this area than what was considered adequate in the past. Most of the time, it is enough. Once in a while, it is not.
Remember, to the gun maker, the case only has to survive the feeding and firing cycle, ONCE, to be considered adequate. A design that feeds, fires, and ejects reliably, even though it may stretch the cases close to their failure point, is still a "good" design to the gunmaker. There is nothing in the "rules" that says guns have to leave their empties in reloadable condition. The majority did, and still do, so we have come to expect this. Today we have several designs that, in my opinion, do not leave their fired cases in safe, reloadable condition, on purpose.
Remember too that gun makers all say not to use reloaded ammunition. In the case of guns with these "undersupported" chambers, might it not be that the designers counted on the use of new ammo?
New brass has the most "stretch" available, before failing. The ability of fired & reloaded brass to stretch (without failing) is less, and gets less each firing and reloading cycle the case survives.
With a fully supported chamber, brass can only stretch in length, and it is this that generally allows reloading (and usually multiple reloading) of the case. These unsupported chambers allow the case to bulge out to the side, and on top of that, the bulge is just in front of the solid case head, an area where the strength and thickness of the case are critical to its safe reuse.
I believe that by letting the case bulge the way they do (with standard safe pressure loads), these guns are essentially "using up" the reloadable life of the case on its first firing. I think the designers intended this, stopping just "short" of the failure point of the case, under normal circumstances.
But sometimes, an individual case will fail sooner than the majority. When this happens, you might get a "ka-boom" from a load (that is within pressure spec,) in the relieved chamber, that would not have happened in a "regular" chamber.
These guns walk close to the line, and sometimes, it gets crossed, without any failure on the part of the ammo maker. Not common, but sometimes, the stars line up, and the entrails are favorable for Murphy to combine things in the worst way.
In your case, an overcharged round is a possibility. A reloaded case (even by a factory) is a possibility. Its also a possibility your gun has TOO large an unsupported chamber, but since other rounds have fired without issue, this is less likely to be the main cause of this specific incident. It could well be a contributing factor, though.
I know many people have, and do reload the bulged brass from these kinds of chambers, without incident. I also know that these cases often fail before identical ones that have only been fired in "standard" chambers. Sometimes drastically before.
I will not reload cases with the kind of bulge shown in the OP's pic. I will not recommend any do so. Your call on that.
My point is that while the likely explanations involving something in the gun or ammo being wrong, are the most possible, one should not entirely discount that everything might have been in spec, and it was the combination of the tolerances of spec lining up just right, this time, to create the "perfect storm" that allowed case failure, this time.