Something new about WP8GDR3 multitasking

I have just discovered something new on my Lumia 520, updated to GDR3, that I have never heard of, so I dont know if it's a (poorly) implemented feature or a bug.

When pressing "back" multiple times for exiting an App (I'm not used to the new way and I still do it automatically, and I'm not sure about what pattern is the triger), sometimes a see a cross on the upper right corner, that closes the App when i press it. I has the same appereance and functionality than the one in the App Switcher, but in appears without enetring in the "multiwindow" mode.



I think that it may be a bug, and it enters and exits the App Switcher quickly, leaving the exit button visible for a second but without the zoom out animation. Do you know why it happens? Has it happened to you? Even if it is, do you think it should be a feature: press back for exit button option, hold for multitask, or something like that?