Some titbits allow us to assume, that containers with dangerous goods, most probably already on fire, were the main reason behind the decision to evacuate all the crew, though fire seems to be quite far from superstructure – as far in fact, as it is possible on board of a ship. And as well probably, those dangerous containers contain some goods which when on fire, emit poisonous fumes.
Tug MAERSK MOBILISER (IMO 9765471) should already arrive at the scene.
Full chronology: http://maritimebulletin.net/2019/01/04/container-ship-yantian-express-on-fire-in-north-atlantic/