Excavator demolishing bridge ends up in river - GIF

[Tooler2]

There was more engineering in that old structure than they gave it credit for

[Frank S]

There was more engineering in that old structure than they gave it credit for

Or less considering the cracking of one section caused a complete domino effect, even the span supports went down.
But the most probable answer is the bridge was much further gone than they had figured. That one collapsed section they were trying to break up may have been a good indicator of the condition of the rest of the bridge