Why is a random app being opened when I exit an app with Back button? On my Nexus 7 G1 (4.4.2 KitKat), sometimes, when I press back button to exit an app, a random app is opened instead of Home Launcher (Nova Prime). Sometimes, this random app is opened temporarily (then, Home Launcher becomes visual). And, this random app isn't chosen from my Recent Apps list. Means, this happens even when the app never appeared in Recent Apps list before.

What's going on? How to fix it?
It's most likely related to how the task stack works. 

In Android, an application can open an activity in another application, but the task stack still belongs to the first application instead of the second. 

For example, if you open a YouTube link from Firefox then the back button will take you back to Firefox. The task stack may often cause apps back button to appear to behave strangely especially if you don't understand how the task stack works; most strange behaviors with the back button is caused by users that don't understand back stack, but sometimes strange back button behaviors may be because of a bug in the application, or because of application developers that don't understand the task stack and tried to work around it rather than with it (causing grief for people that do expect proper back stack behavior).

The task stack is also the reason why the Recent apps list may show an app that belongs to say Firefox but shows a Screenshot that appears to come from the Youtube app, and clicking that app will bring you to a Youtube activity instead of to Firefox.

The recent apps list shows both the app name and a screenshot because both are needed to decide which app to switch to. If you want to go to the Youtube app to start a new task, you should switch to the app that is named Youtube, instead of any apps that have the Youtube app screenshot.  But if you want to return to the video you watched before that email come that interrupted you, then you want to go to the one that shows the right Screenshot, which may actually be named with your browser or newsreader.  

I used Firefox and YouTube as an example but this works similarly in any properly written apps.