Actually, one of them did start to lean before coming down. The outside vertical structural members can be seen blowing out, the top section leans, and then it comes straight down.

The came down the way they did, basically, because on one the floor structure weakened as it heated up and then the outer members snapped, and on the other the central core lost strength as it burned.

The fire was mainly from the contents - the fuel from the planes acted like charcoal lighter fuel.

And they were not designed with coming down in mind.

What I've never found anything about is why an associated 47 story structure collapsed. I'm sure there's a reason, I've never read anything about it.

Arf