Thaaat's gotta suck...
I like this clip for a number of reasons (besides the obvious).
Yes basically, the mortar is a pretty simple weapon... to fire. If you want accuracy with it, well that's another thing, but just getting it to go 'boom' is really pretty simple. Make sure the safeties are removed from the round, drop it in the tube, it falls to the bottom and hits the firing pin and - well, the rest just kind of comes naturally.
If you're the type to skip out on aiming stakes & sights and go by the tried and true 'spray and pray' method, well, you're probably also the type to take a pass on cleaning out the bore every now and then.
Mortar barrels have been known to get dirty, just like rifles and definitely not like that floozy showing off too much ankle three hovels down from your own.
When barrels get dirty, rounds can (will) get caught up in the tube. When rounds get caught up in the tube, I'd imagine that more likely than not they're not going to explode in the barrel (the explosion in the barrel comes from the propellants attached to the rear of the round), but all that pressure's got to go somewhere. (This is that same pressure that shoots the round up to the neighborhood of 3000 meters.) If the pressure can't go up, it'll go down and probably either find that faulty and/or loose firing pin or it'll definitely find that barrel failure that you didn't see upon inspection of your weapon. Dumbass.
The fact that the video existed to post up for posterity leads me to belive that the round didn't spew shrapnel all over the place turning the guys into meaty man-chunks (to include the camera man and his camera), but, Inshallah, the failure of the tube was enough to ensure that all in the immediate vicinity had a Very Bad (read: slow & terminal) Day. Further viewing of the clip shows the barrel failure (flash at the bottom), resulting in it getting some air time (lovin' it!), suggesting that the round was almost definitely stuck in the barrel and the ass end of the tube was hopefully embedded in the tender bits of nearby schmucks. If not the bits of the tube in the bits of the guys, I'm sure the sand bags, base plate, rocks and other assorted items were moving fast enough to make things interesting...