Looks like some of those containers exploded, which would be why 40 fell off and more are now on fire.

Edit: after reading the article, I remembered some shipboard firefighting training from the Navy. A "reactive flammable material" is one that "explodes" when water contacts it. So the fire fighting teams have to spray the surrounding area to keep it cool enough from starting a reaction due to temperatures. It would also explain why the crew was removed and the use of fire fighting tugs. Usually, those types of chemicals will continue to burn until fully oxidized. They also burn at temps that humans can't physically handle. Temps usually over 1,500° F. Magnesium and thermite are types of "reactive flammable" materials.

Why would a sea going vessel be transporting such a hazardous chemical that ALSO violently reacts to water? Why would a mining company use it?

Looks like some of those containers exploded, which would be why 40 fell off and more are now on fire.