Tough question . It could be a malfunction of the back lighting panel. Some back lighting is edge , local or full array.
Part of the system could go bad in an isolated area.
The SONY model 930D , has a dual light panel that seperately could go wrong , either the top or bottom. It's new technology for Sony tvs to increase brightness and overall picture quality.
An HDMI cable could have a glitch that more than likely would affect the whole pictue , but anything is possible.