Possibly (probably?) a coincidence, but it did look like broad changes to how reading was taught started to land in 2010-2012: https://www.edweek.org/teaching-learning/common-standards-dr...
The real culprit is probably more in line with far more alternatives to reading for entertainment.
Not a rigorous response, but Minecraft.
Kids were playing a ton of online games before Minecraft.
Yes and no
Up until the 2010s I think it was still a lot less socially normal to play a lot of games. We reached a tipping point somewhere that went from gaming being a sometimes activity for kids to basically every kid plays games
Most of the people in my high school in the 2000s didn't play games as a primary hobby. Only a few of my friends had a PC for games or a console. It wasn't nearly as ubiquitous as it is now.
The release of the Xbox coincided with what I viewed as a big jump in video game players. Even the biggest athletic jocks played a ton of Halo. And then not long after that saw much older people start playing games like CoD:MW as after work entertainment instead of just tv.
YouTube.