woofers gone wild

Hi folks,

My new Unison Unico amp sounds fantastic, but there's something slightly strange going on that I've never seen before.

When the amp turns on after its 20 second slow start, the woofers on my speakers make a popping sound and recede about a centimeter back into the cabinet. What the heck? Is that normal? When I switch black and red on the back of the speakers, the woofers pop outwards. Either way, the woofers pop back to normal when I turn the amp off.

Any clues as to why this might be happening?

Thanks!