Just eyeballing those graphs, the striking thing is the difference from the beginning (early 70s) to the peak in 2012. During that period, reading scores only increased by 8 points while math increased by 19 points.

I'm definitely in the camp that thinks cell phones have something to do with what's happened since 2012. If we start from the iPhone in 2007, it seems plausible to me that a 5-ish year lag is consistent with the time it took for smartphones to rise in popularity and their effects to filter into society. (I got my first smartphone around 2012, by which time pretty much everyone else I knew already had one.) That's at least a gesture toward explaining why there was a peak around 2012. What it doesn't explain is why the ascent to that peak was so much steeper in math than in reading.