Here's a preliminary explanation/guess of what happened from Reuters:
Trade publication OPIS, citing an unidentified source, reported that an electrostatic precipitator (ESP), which reduces fluid catalytic cracker particulates, exploded as contract workers were doing maintenance on the nearby fluid catalytic cracking unit, or FCC.
"Contractors working on the FCC to fix the expanders," the source said, adding that an injection of ammonia on top of the flue gas stream caused a pressure buildup, which resulted in the ESP unit explosion.
The unit could take up to a year to replace, the source said.(I confess that I don't understand refinery technology enough to know exactly what that means.)
It's likely a coincidence that the United Steelworkers union has decided to go on strike recently at a number of oil refineries around the country. That said, I think it definitely highlights their emphasis that this strike is about worker safety. I don't know enough of the issues to make a judgment, but that seems like something just as important as wages/benefits.