I’d chalk it up to how the two games work. D3 only ever really kicked me out of the instances game I was in if I was idle too long. Usually wouldn’t kick me out of the entire game though, just to the character screen where you can choose what to do. I want to say it took a good while though.
D4 works similarly but may also have a faster internal timer of when to kick people. More than likely they never adjusted this since the launch of the game. Sadly without seeing it first hand I can’t really say.
I also can’t account for just random disconnects either. Although seeing as this only seems to happen when you’re alt-tabbed I’d just chalk it up to D4 being weird.