A pressurised tank (110 to 165 psi) ruptured, causing the accident. The chemicals involved are NaOH and sodium sulfide (white liquor) which is what is used to turn wood chips into pulp - and the source of the H2S smell that comes from paper mills
Note: this is in Washington state not California, and is unrelated to the incident in the news the last few days, with people being evacuated in Orange County. This seems to be at a facility that creates paper products out of lumber.