This is the result of using a flash or studio strobe that is not synchronized with the fast shutter speed being used by the camera. The issue is common with all focal plane shutter cameras. It occurs because the shutter is passing the image sensor too fast for the flash to sync with it. To resolve this issue, slow down the shutter speed.
IMPORTANT: When using the SLT-A99V, ILCE-7, ILCE-7M2, ILCE-7RM2, ILCE-7S or ILCE-7SM2 and shooting at a shutter speed of 1/250 second, use a flash manufactured by Sony. When using a flash from other manufacturers, adjust the shutter speed to 1/200 second or slower.
NOTE: The flash sync speed of most cameras is generally between 1/125 to 1/250 second, depending on the camera. This means a flash cannot be used with a shutter speed faster than this, unless the flash has the High Speed Synchronisation feature.