The theory was that the devices had a propulsion system that ionized the air around them, making them looked blurred.
This is coming to light more and more. The recent “tic tac” footage shot by military pilots, also show this “halo effect”. Researchers are actually going through footage and pictures because there is a belief that this effect can distinguish between real and fake sightings. However, this effect does not hold true for all cases, for example the Arizona Lights that occurred over Phoenix that thousands witnessed.
If we are talking about the same "Phoenix Lights", that was proved to be a military exercise - the aircraft dropped flares which appeared in sequence and went out in sequence. If there are others, I haven't heard about them.