USCSB published their findings from the explosion. Wild it was 5 years ago - I remember it waking us up in when we lived in Midtown, and my neighbors said it broke windows
TLDW: an old propylene line gave way, started leaking, and leaked continuously into an automated working area. Watson Grinding had previously disconnected their gas detectors from their computer control system. So there was nothing to stop the building from filling up to stoichiometric levels and to cut power to the building. Facilities DID actually send out a warning notification to workers, but the first person in for the shift didn't hear any alarms or look at his messages, and turned on a light switch that ultimately was the detonation ignition.
The kicker? Watson seemingly did not break any statutory rules or safety standards as they weren't required to create a process safety standard, but USCSB argues adoption of those mechanisms would have prevented the explosion.
Ultimately, Watson did not maintain their safety equipment and operated a facility that did not fail into a safe state. The facility was designed with fail-safe measures, but they had been disabled years prior to the incident. Contractors had raised concerns in writing to Watson in 2013, 2016, 2019, and just weeks before the explosion.
Watson was replacing copper lines with rubber hoses, and had disconnected their safety equipment. Thank god for our business-friendly state!