The most probable cause is cheap LED lighting, maybe even using RGB LEDs.
As with most shutterless cams, the image information is read from the sensor in lines. This causes two known effects: The Rolling shutter effect and banding.
What you see here, seems to be banding. To dim LED lights, you have to pulse the light. Your eyes are slow enough not to notice these pulses, but the sensor can see them.
So basically you read some lines from the sensor, then the LED turns less bright, then the next lines are read, then the LED switches on again... and so forth...
What can you do about it? Either use specialized video/photo lights that switch fast enough for the sensor. Or try to even out the pictures in post, which is not an easy task.
A good test if it is related to the light source, is doing a test shot without any artificial lights on and check if the effect still occurs.