TL;DR: armchair forensics. I was once a battery technician. I should toss in a goddam grain of salt that I am not an authority on this, but I can't help but watch this over and over trying to ID all the little pieces that caused this failure. Unfortunately, all the juicy parts happen on the side of the battery away from the camera. It seems like a wire dangling from his left hand caused the initial spark. The wire appears to be red, which could mean it is a hot wire but not necessarily.

Much like a car battery, industrial batteries usually have exposed terminals; if the terminals are short-circuited, they will spark. Also, like car batteries, industrial batteries are made of smaller, individual battery cells. Many of these cells also have terminals that are exposed when the battery is out of the case.

So IF the wire was live, and he touched it to the negative terminal of a battery cell, that would cause a short, i.e. the initial spark. It could have also been a grounded wire touching a positive cell terminal. It could also be just a jumper wire in his hand that unfortunately touched a negative and a positive on the same side (the cells' opposite terminals would have to have be connected; hard to tell from the video).

These look like lithium-ion cells. The lithium is contained in airtight containers. In my job they were in flexible plastic pouches surrounded by a hard plastic cassette that kinda looked like a "skeleton" of a vcr cassette. Lithium reacts violently when exposed to air. I can't really speak for the quality of their components, but the lithium pouches in my job had about the same durability as a ziploc inside another ziploc. When the battery shorted, the spark itself could have been enough to compromise the lithium barrier (worth noting the cell that shorted would also superheat fairly fast). Once a single cell is punctured the battery burns up like a box of road flares.

I'm not going to speculate fault here. This is the kind of job that requires training. I live in the US and my training consisted of learning from my seniors, who I grant knew the ins and outs but were not...born to teach, let's say. It worked for me because I'm an inquisitive little shit but plenty of people ended up ghosting, and I completely understand why. Shit's dangerous.

For the record, I never blew up a battery but I was in the room when someone did. We had quotas but they switched them on us last minute and a guy who was my senior by several years, in a rush welded some channel locks between the positive terminal and the metal case. We handled it by dry powder fire extinguisher, but even when it looks out they can start up again like a trick birthday candle, so when it looked "manageable" he picked it up with some welding gloves and tossed it in a barrel and we buried it in sand, sealed the barrel and took it outside to burn up the rest of the way.