Local media report that he [the diver] was killed by an explosion while using an underwater oxy-hydrogen cutting torch. Dutch marine news outlet Schuttevaer reports that Huijben was working on removing Bayesian’s boom. The team initially attempted to unbolt it, according to the outlet, but when this failed they opted to burn through it. It is believed that an explosion - possibly from hydrogen bubbles - caused a piece of metal to break loose and hit Huijben.
According to commercial diving historian Francis Hermans, the risk of igniting unburned hydrogen gas from an oxy-hydrogen torch has been known since at least World War II, when U.S. Navy salvage divers found that there could be difficulties with highly explosive gas accumulation in enclosed compartments when working inside of wrecks. This was reportedly among the reasons that Navy operators turned to the oxy-electric arc torch, which requires only oxygen in its gas supply and introduces no flammable gas into the working environment. Most commercial salvors followed suit in the following decades, according to Hermans.
There is an interim report on by the UK Marine Accident Investigation Branch just out on the disaster–which is more complete than the newspaper accounts we have been discussing: