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Why were there reports of explosions from the buildings?

Because huge buildings beginning to come apart make loud noises.

One of the unfortunates marooned in an upper floor of WTC1 sent a Blackberry e-mail message a few minutes before it collapsed, telling his horrified correspondent that they could feel the floors' creeping distortion and flexing under them (they were flat on a conference-room floor, trying not to inhale smoke), and they could hear the steel structural braces on the floors below them pulling free from the outer box-beams and crashing down.

Why did the Towers fall so fast in freefall and explode outwards rather than like the "pancake" theory?

If you look at the video of WTC2 beginning to fall that was taken from about the SE corner of the property near the Marriott Hotel, near the pedestrian overpass that survived the collapsing buildings, you can see the pancaking going on inside the tower even as the skin peels away and falls outward (mostly, some collapsed inward) from the upper floors after the collapsing mass has passed by and stripped away the lateral support provided by the floors and their support braces.

As the collapse mass reaches each floor and it collapses, a puff of dust blows out the windows on all sides as the floor collapses violently onto the floor below, taking it out. Survivors of the collapse who were in the stairwell said they could hear the collapse-mass coming (God, that must have been an experience utterly beyond everything that we call reality) and that it was a tremendous pounding noise that they noticed most, before the mass reached them and passed by on its way to bedrock. So the building really did pancake.

The upper floors collapsed differently, beginning to fall as a cohesive mass and then disintegrating in flight as stresses far in excess of design simply disarticulated the structure, grinding up its contents into a cloud of falling dust, beam segments, and paper. Someone who claimed to know wrote afterward that the largest piece of office furniture recognized in the wreckage was half of a telephone keypad. So he said.

The engineering study of the collapses also noted other significant differences in the demise of the towers, the chiefest being that WTC 2 fell because the outer box-beam skin failed where it had been damaged by the airliner, whereas WTC 1 failed first in the central structural mass.