This is not bias. You don’t know in advance what the cause of an event is for sure. People mentioned a probable cause (violent homeless) and it happened to be a less probable one (a tech co-worker), although we still don’t know for sure yet. In particular, the influence of drugs and mental illness among the homeless were thought to explain the event.

It’s like predicting the price of a stock that didn’t come true. Not every incorrect prediction is due to selection bias.