Is it the threat to higher skill jobs? Because it seems to me quite normal: since it began automation has taken over progressively more skilled tasks, starting with the least skilled labour.
Is it the threat of higher education/skill requirements for jobs? Automation, by taking over the lowest skill jobs, both creates higher skill jobs overseeing and creating the machinery and pushes the labour pool to a higher average education/skill level. Is there a critical point we're reaching? Will university education become required?
Is it purely about numbers? The aforementioned process of automating low skill jobs while creating higher skill jobs includes within it a general reduction of jobs (leaving aside the creation of new job markets like entertainment or information workers). The low skill jobs are the most numerous and the newly created jobs are much fewer. Why have we been able to keep people employed despite automating away shovelers, weavers, and almost all other manual labour? We even have a much much larger population since the industrial revolution began, what process created jobs for everyone?
Because we don't have full automation. Take construction for example; it was all planned and built by humans, and it still is, we just use better tools to plan and build things. We don't need to weave things by hand, but we still need people to (sometimes) operate the machinery, invent it and maintain it. I remember reading an article not too long ago about an Amazon shop with no staff, and it works by using sensors to automatically charge your card a certain amount when you leave with the stuff you want. The irony is, to create a shop with no staff, it probably took way more people to work out and implement that than it would to just operate a normal shop.
However, I think the difference now is we are approaching a point where the amount of jobs will start to fall in developed countries. Not just low skill ones either. Machines just do certain things better than us, and the number of things they do better than is is going to keep growing because our potential is capped and theirs isn't.
Automation was very primitive during the industrial revolution, and people still had to operate everything. On top of that, we suddenly created an entirely new field of work - creating tools to automate things. I would argue that we are probably still in the primitive stage of automation, although it is significantly more advanced and commonplace than it used to be.
I think this is being recognised by some countries now because they are having trials of universal income - paying people for no reason other than to give them money. There has been some criticism of this, but I really don't see what we are going to do when we get to the point where 20% of our population has nothing to do. It might not be a bad thing though, as it is simply going to change the way we think about work. Instead of it being a necessity for financial reasons, the reward system will be different. People will do more things because they want to do things. Some people might argue that there has to be a financial reason to work, but there are already examples of people who do things for no other reason than wanting to do things, such as musicians, artists, certain religious activities, some sports etc.