The full description is with the video on YouTube. Gives a good technical explanation of what happened.
The quick summary is that the jumper bought the rig used. It was an old rig from the early 2000s with a known safety issue. The rig had been modified to address the safety issue, but was modified incorrectly. This incorrect modification caused the malfunction.
This would be the equivalent of buying a Cessna from the 1960s which had some kind of AD issued, the AD was not complied with correctly, the new owner either didn't get a prebuy inspection or the mechanic doing the prebuy inspection didn't catch the improper repair, then as a result, the wing breaks off in flight one day.
Do these things "just happen?" Yes and no. They happen about as frequently as wings fall off airplanes. Super rare and usually after a lack of maintenance, inspections, and/or training. It's not much different from flying.
If you use modern equipment, inspect and maintain it properly, get good training, and use good judgement, it really minimizes the chances of something like this happening. Start cutting corners on any of those areas and, ehhh...you never know. Might be ok, might not.
Skydiving has truly been a life changing experience for me and I wish more people got to experience it. Can't wait to get jumping again after the 'rona fades away.