Googled a little bit and this is what I found:

This illusion works on a principle involving a trait human eyes have: the color sensitive areas actually get “fatigued” if they stare at an area with the same color for too long. As a result, you see the complementary color once you take your eyes off that area. In this case, because the pink dot at one position is there for all but one time when the animation has it disappear, you see the green dot there because you’ve stared at the pink for too long. Simple. If you stare at the picture for a while, then look away, you should see aring of green dots for a split second. Same principle.

he disappearance of all the pink dots is due to the brain’s trying to take a shortcut, and ends up combining the inverted pink with the real pink, leaving nothing but the grey background.