I know about this illusion, it all happens on the defocus on simple blending things, like you're seeing the hard + (Plus

) Sign, you feel someone's actually operating the appearance and dis-appearance of the pink objects, it's all because of the grey background which betrays' your eyes, and it's a simple, but long issue..
go to paint, PS, whatever, change the colour of the objects (pink ones') to any colour, you'll get the same result as you've got with the pink one, and try changing the grey BackGround to white, see what happens.

........
EDIT: !Also, don't see it for too long, after you're done seeing it, close your eyes and keep the fingertips on your eyes and move your eyes around a bit, other wise it can cause a lot of headache..