I think if the boiler doors were closed then the air inside the boiler rooms might not have been able to vent out. As the ship broke near the engine room there may have been an attempt to seal or batten down the hatches around that compartment which led to it compressing and bursting open with a huge explosion. Survivors saw large plumes of smoke, sparks, and coal shooting out of the funnels. I think there had to be terrific compression inside to cause that effect. e.g.
Mr. Osman
"After she got to a certain angle she exploded, broke in halves.....You could see the explosions by the smoke coming right up the funnels.....It was all black; looked like as if it was lumps of coal, and all that......pretty big lumps.....Just after the explosion....through the funnels......steam and very black smoke."
Mr. Mellors
"There was suddenly an explosion and I found myself whizzing through the water at an awful pace, having been blown away by the explosion"
Mr. Weikman
"The explosion blew me along with a wall of water.....There was a great number of people killed by the explosion."
Mr. Hyman
"There came a terrible explosion, and I could see men, women and pieces of the ship blown into the air from the after deck. Later I saw bodies partly blown to pieces floating around, and I am sure more than a hundred persons were blown off into the sea by that explosion. A terrible hissing of steam began and the awful cry went on. I tried to close my ears, but there was some mysterious attraction and I had to hear that cry.....The hissing and screaming kept up, and finally the ship seemed to right itself."
Philip Mock
"After the noise I saw a huge column of black smoke slightly lighter than the sky rising high into the sky and then flattening out at the top like a mushroom."
Mrs. Collyer
"Something in the very bowels of the Titanic exploded and millions of sparks shot up to the sky, like rockets in a park on the night of a summer holiday. This red spurt was fan shaped as it went up, but the sparks descended in every direction in the shape of a fountain of fire. Two other explosions followed, dull and heavy, as if below the surface. The Titanic broke in two before my eyes"
.