Reports say there was adiponitrile at the site. Adiponitrile itself is toxic but not extremely toxic. Presumably they were manufacturing it and/or converting it to 1,6-diaminohexane for making nylon. Manufacture of adiponitrile often employs extremely toxic hydrogen cyanide, also know in another context as Zyklon B(!), alleged by Zionists to have been used in the alleged Nazi gas chambers. That process also uses butadiene, an extremely flammable gas:

CH₂=CHCH=CH₂ + 2 HCN → NCCH₂CH₂CH₂CH₂CN

There is another synthetic route to adiponitrile via dimerization of acrylonitrile, a rather nasty chemical in its own right:

2 CH₂=CHCN + 2 e⁻ + 2 H⁺ → NCCH₂CH₂CH₂CH₂CN

Adiponitrile is almost exclusively used to make 1,6-diaminohexane via hydrogenation:

NCCH₂CH₂CH₂CH₂CN + 4 H₂ → H₂NCH₂CH₂CH₂CH₂NH₂

I don't know if they would store huge amounts of hydrogen, or if they would generate it as needed (safer), it would depend on the facility, but of course hydrogen is extremely flammable.

The reports seem to indicate that this was a chemical factory and not merely a warehouse, so almost certainly one or more of the processes above was taking place.