This is a strange malfunction you don't see very often — or ever. While his main canopy was deploying, this skydiver's left riser disconnected, and — his RSL being connected to that side — it partially pulled the reserve. He attempted to cutaway but couldn't get the handle to budge. He threw the reserve free bag to the side as far as he could, and then used both hands to pull on the cutaway as hard as he could. The second cutaway attempt successfully discounted the right riser, then he dealt with the line twists in the reserve. Notably, he was holding onto the right riser when he chopped and consequently broke a finger. Fortunately, that was the extent of his injuries.
The jumper who submitted this video said that he spoke to multiple master riggers and that both the canopy and rig manufacturers are investigating this incident. At the moment, the cause of the incident is unclear. The master riggers said that "based on the damage patterns... everything was hooked up correctly." Some theories were presented but there is no evidence to support them and rather than publish unverified conjecture, we are going to just leave it at that.
The reality about skydiving, or any inherently dangerous activity, is that no matter how good the equipment is, or how experienced the operator may be, something can still go wrong.
In skydiving, the factor that causes dangerous situations is almost universally the operator. Most of the injuries and deaths we see are due to user error. Examples include packing mistakes, poor decision making under canopy, ignoring safety standards, not doing gear checks, etc. However, sometimes — rarely — there are unpredictable equipment issues that the operator could not have prevented. Those situations are the ones where emergency procedures really matter and when having trained yourself to stay calm in an emergency can save your life.