This is a catch-all category for types of glitches that are extremely loud. These are caused by major disturbances in the LIGO detectors, for example an actuator reaching the end of its range and 'railing,' or a photodiode 'saturating.'
In time-frequency plots these glitches will look very bright, meaning that the glitch has very high energy, saturating the majority of the image.
Cause:
Major disturbances to the detectors, various causes.