When you touch the screen in two places, the screen will interpret them as a single touch, in the average position of your two touches. So, touching the two edges of the screen is seen by the device as a single touch in the centre of the screen. Releasing one thumb is then seen as a (correct) single touch at the point where your remaining thumb is. Thus, what the device sees by this is your touch swiping instantly from the centre of the screen to one edge, which swipes to that edge's desktop.