I heard it was a rare foundation for such a tall building. Additionally the building is made of concrete making it even heavier putting excess pressure on the soils below.

That question is still being debated.

The 2 leading theories are:

1) it has a floating foundation (in sand, not to bedrock), unique for this size of building in SF.

I talked to an architect once, and he acted like this would be ok. Guess not.

2) workers on the nearby tunnel "dewatered" the soil near the building. Changing the moisture level changes the resistance of the sand to weight.

Note that in an earthquake zone, soil "liquefacts" and settles, forcing water up, so it's terrifying that a large building rests on that. The building could literally plunge into the ground, or fall over during a liquefaction event.

See the videos from Kobe - waterfalls appear on top of the ground in liquefaction areas.