High pressure makes bolts melt faster.

 Hi  Hanson,  People tend to ignore the pressure on the bolts.
The jetliners were heavy, with extra-strong momentum, diving full-throttle.
High pressure makes bolts melt faster.

 Hi  Uncle_Al,

Pounding hot pig iron, or hot cast iron produces ( much softer ) wrought iron.
Likewise, bending metal ( applying pressure ) weakened the steel.

So the jetliners' full-throttle dive into the buildings weakened the bolts.
And that's --> Before <-- any heat was applied.