This is nothing new, and has had occasional appearances (when needed) for...I guess about 3 years now (something like that). Please note that Alasdair even says "once again..." when introing it.
As for the reasons, well, as he says, go click on the spoiler tags and find out why (in this particular instance) we felt the need. Also note that he/we did not give spoilers, just noted that there were elements of the story that might be problematic for some and left it up to the individual to check them out.
These will be applied when we feel the need - certainly not as a standard "one size fits all" to all stories.