[petek]

We noticed the same thing in Philly (okay, Sarah noticed it and pointed it out to me). After extensive research, I have postulated a theory that it is due to a resonance between gravity waves and the color purple. The resonant peak of this interaction is at about 400 nm (hence, it mainly affects the purple balls) and it is a very weak force, which explains why it manifests itself on curved, light-weight objects such as poof balls.