It looks like the piece of the step that’s missing has been gone for a while - is that dirt or oxidation in the area where the aluminum was cracked/broken off? When aluminum or any metal is exposed like that it’s bright and shiny. Kind of odd it’s dirty or oxidized - it could have been that way a while. It also appears the same oxidation or dirt is on the inside of the post where the step sits.
It looks like the step had been broken for some time before it was loaded with enough weight to make it fail.
Like
@Vtbow said, any number of things could have caused that - throwing it in truck, dropping on concrete, etc.
If not and it was just a bad step, hopefully it’s a reminder to everyone to inspect your equipment carefully - the fall could’ve been avoided if the piece of material was missing before the climb, and noticed.