According to this, the "the blaze started during maintenance work on the titanium roof". So my estimate would be someone was doing hot metal work without sufficient precautions.
Hard to know how much of the roof was opened up for firefighting versus from the maintenance work, but these photos may tell a little more of the tale.
If I recall correctly, liquid titanium is spontaneously combustible in air...
Edit:
As a powder or in the form of metal shavings, titanium metal poses a significant fire hazard and, when heated in air, an explosion hazard. Water and carbon dioxide are ineffective for extinguishing a titanium fire; Class D dry powder agents must be used instead.
-From Wikipedia
Hard to know. Could be just that sparks initiated a fire below the layer of titanium cladding and whatever insulation or substrate was used was in some way combustible. Doesn't look like any updates thus far as to the cause of the fire.