why aspect ratio is changing?

Watched our first blu-ray dvd last night on brand new pani bd-35 dvd player & brand new pani plasma 58" tv on thx mode. Great treat both in sound and vision. While watching The Dark Night the aspect ratio kept switing from letter box to full screen depending on the sceene. Connection was hdmi direct from dvd to tv. Can anyone help me understand why this happen. Thanks T. J.