Nobody knows! What is the official government theory behind the collapse of building 7? Unknown.
Official Government Report
“The specifics of the fires in WTC 7 and how they caused the building to collapse [“official theory”] remain unknown at this time. Although the total diesel fuel on the premises contained massive potential energy, the best hypothesis [fire/debris-damage-caused collapse] has only a low probability of occurrence. Further research, investigation, and analyses are needed to resolve this issue.”