I have seen this exact same thing happen before. Many of the streetlights around here are in blocks, controlled by a PC for each block and a contactor. What I think is happening is the contacts on one leg of the contactor are pitted caused by arcing from normal operation. As the contacts arc and pit, material is burned away each time and eventually it gets to the point where it barely makes contact, sometimes intermittently like in the video, or not at all (total failure). Or it's possible the voltage going to the control relay coil for the contactor is having a problem, bad connection-bad ground, causing the relay to rapidly open and close the contactor contacts, or "chatter" like in the video. I see this all the time with motor contactors. The same kind of thing happens just from normal operation of the high voltage contacts opening and closing, arcing and pitting. Also the same kind of thing happens when the relay coil has voltage problems. It could also be that the relay coil is failing as well.