Theories

Submarine ?
Considering the time table of the sea war it is less likely that Mary was intercepted by a German U-boat. Some weeks later, yes, but not on the 16th-18th of February. Furthermore, there was a storm on the 17th, probably preventing hostile action from submarines.

Sea mine ?
Did she hit a sea mine. I have to examine this possibility further.

Weather ?
There was a very strong storm on the 17th. This may be a more probable cause (or contributing factor) than an act of war. A sister ship of Mary's was downed in a storm in the Bay of Biscay.

Competence of crew ?
The officers were experienced. But almost half the crew was replaced in Helsingborg on the outward voyage. They were new to the ship and many were very young. Could that be a factor?

Sea worthiness of the ship ?
Mary belonged to a family of thirteen sister ships. Twelve out of those met with disaster.

Fire on board ?
I understand that coal cargo carries a risk of self heating leading to cargo fire. I must follow up on that trace. Read more.