What causes speaker cone to extend when...

Recently I made two changes to my audio system and each time I noticed the speaker cones made several large extensions while playing music before settling down (that is, the whole cone moved in and out from the rubber surrounds). It was almost like they had a hiccup. The music was playing and everything sounded OK. The first time was when I disconnected my speaker cables from the amp - everything was turned off by the way - and replaced with new cables. The second time was when I disconnected my turntable, made some changes, then plugged it in again, along with new phono interconnects.

I was wondering what could cause such a reaction; e.g., could it be new voltages moving through the system?