Lu,
to have a fire you had to have fuel, Oxygen and a fire source. Where was the fuel that would result in such a violent explosion?
Well the thing is, if you take Hydrogen as a gas and light it, it will go bang.
I know you'll correct me if I've overlooked something, and in my naivety as a young student (!) would like to know!! but, Oxygen behaves in some similar ways to Hydrogen. Hydrogen burns in air because there is oxygen present. The hydrogen is the fuel. Liquid oxygen is inflammable and behaves as a fuel source, try burning it, well.. ok I wouldn't reccomend that but still.
I guess I'm trying to say that with the presence of oxygen in the air and a fire source, liquid oxygen will burn. As the energy of the oxygen increases (in micro seconds) it returns to a gaseous state where it expands very rapidly producing the explosion.
Any chemists set me right!!
Andy.