It started when cell phones with texting became inexpensive enough for parents to buy them for their high schoolers.
Many started staring when they realized they could stop wasting time by thinking. Why think when I have a device that will do it for me, they might ask.
The culling phase will eliminate many of these folks as they cross the street or drive while not thinking about whay they’re doing.
Another datapoint (in a rather depressing article) Have Smartphones Destroyed a Generation? - The Atlantic