Soundstage moving forwards

Hello,
One of the things always intrigued me was how soundstage moves forwards when I play music louder. When playing at very low level, it is deep and somewhat narrow. As I increase the volume, it gradually moves forwards and at 95 dB peaks, it is almost at the plane of the speakers. The depth of the stage also seems flat at that point. What is the explanation? I have standard cone drivers. Is it limited to cone drivers or seen with electrostatic and other types of drivers?
thanks,
Giri