I would think you may have inadvertently clicked on a bookshelf speaker ad and the search engine then flagged that you looked at bookshelf speaker and then started barraging you with bookshelf speaker adds. If you triggered and interest... then it can pop up all over. Once you notice it, then you start noticing them... like you buy a car you never thought you would buy... and suddenly you see them all over the place.
I haven't noticed any increase in bookshelf speakers being highlighted or more common. They have always been common because because they can be cheap... and of course over the last 20 or 30 years because of PCs.