The bottom tread had a worm infestation which resulted in the tread collapsing, being the bottom tread it takes the most punishment over the years, the infestation was not too serious but had travelled to the second tread and riser, once a proprietary treatment had been applied a replacement piece was made and fitted into the bottom tread. If you suspect you have a worm infestation you can determine if it is active or not by laying a black piece of card under the affected area, or if it’s a piece of furniture place it on top of the black card.
Then wait about a month and see if you can see really fine sawdust. It will be as fine as talcum powder, that’s the result of active woodworm if you see none then its historic rather than current.
When you do this you must ensure that you do not disturb the area or piece of furniture, vibration can disturb loose debris that is historic and give the impression that it’s a current infestation.