[frog in a pot]

...but allowing that size of a pre-stressed concrete beam (950 tons) support its own weight without temporary shoring was the cause.

The absence of temporary shoring or the cable support system suggests someone calculated the walkway could support its dead load. Just as bad, if it was cast-in-place the shoring may have been removed prematurely.

Or:
"Dude, go ahead and place the walkway, those cables are just for cosmetics."

[CedarDave]

Just as bad, if it was cast-in-place the shoring may have been removed prematurely.

Concrete does not reach maximum strength until sometime after pouring:

One criteria used is the 28-day test strength as the allowable stress value for the design. There are high early strength concretes that can give you quicker curing times but 28 days is the norm.

For many purposes, concrete can be used or loaded at earlier times depending on circumstances. Normal concrete will get about two thirds of its strength in just 7 days so many times that is the criteria for some uses.

How long does it take freshly poured concrete to cure to full strength?