Thanks for the link. The video I saw on TV was of longer duration than anything on the YOUTUBE video. It shows lights appearing in a sequence from right to left across the screen. Multiple flares dropped from an aircraft deploy their parachute and ignite at spaced intervals. If the flares are 10 or more miles away they will appear, for a short time, to hover even though they are descending. The YOUTUBE video only shows brief snips of the footage shown on History Channel which I have no doubt were air-dropped illumination flares.
It may be that there was a different triangle-shapped phenomenon people in Phoenix have reported aside from these flares.
There were supposedly two separate sightings in Phoenix that night. The first is the one you describe, of illumination flares in the distance falling behind a local mountain range.
The second happened some time later (an hour or two, I believe). This is the one where people say they saw a triangular object flying at very low altitude over their houses in the city.