Was the March 2019 Rafale jet ejection incident investigated?
Here are a few key points translated from the media coverage on the official BEA report (PDF, in French):


The passenger was a 64 years old manager of a shipping company.
The French Air Force carries out such passenger flights occasionally.
The passenger wore a pulse watch which recorded between 136bpm and 142bpm prior to the flight, safe to say the man was under a lot of stress.
The report states that the situation produced significant peer pressure on the passenger: A significant number of colleagues from work, who had organized the event, were present.
In order to max the element of surprise, the passenger passed the required medical exam the same day. (As a consequence of this accident, there is now a 10 day mandatory delay between the exam and the actual flight.)
The physician issued the medical certificate under the condition that the flight must not exceed 3G. This information, however, did not reach the pilot due to what the report calls a "technical problem". (This type of aircraft reaches close to 4G during a normal takeoff.)
The passenger wasn't correctly equipped: The anti-G trousers as well as the chinstrap were too loose, the helmet visor folded up. The passenger lost his helmet mid air after the ejection. 
The passenger ejected in a phase of the takeoff with negative G forces.


Most likely, the passenger was under an extraordinary amount of stress and tried to hold onto something in negative G, unfortunately the eject cord.

There's, however, another not so fun fact:

The pilot should have been ejected automatically along with the passenger. This mechanism failed! The pilot could have ejected himself nonetheless, but chose not to. It was due to this technical malfunction that the French Air Force didn't lose such an expensive machine.

(sources: Le Point, BFM TV)