Several eyewitness accounts relate how some ufos, or the energy surrounding them, seems to almost explode particularly when they hover in mid-air.
In one account in an 'unannounced' visit to a remote desert area of S-4 (Area 51), on a Wednesday night in March 1989, Bob Lazar observed a white-light craft ascend into the night sky, hover motionless and then undergo "impossible flight characteristics’, such as instant 90-degree shifts in direction, and instantly re-locating to another space.
But then Bob also said, "At times the craft had glowed tremendously bright and we thought, being miles and miles away, we thought there was something wrong and the craft was going to explode – so much so that we all got behind the car." (see Bob Lazar's video for more details - but first see Jon Farhat's excellent Bob Lazar website).
Gene Huff, a friend of Lazar's who witnessed the same thing, gave his account of the same event, "It came down the mountain range toward us to where we could actually see it was elliptically shaped. It looked like something that, its strange to explain, it looks like an explosion that starts to explode and stops and doesn't explode and dissipates. That's how bright the light is, your brain interprets it as an explosion about to happen".
[Both of the above accounts are from "Return to Dreamland" (UK) tv program]
Another account comes from "Janet’ a witness to Linda Cortile's 1989 abduction from her Brooklyn Bridge apartment, "First [the ufo] looked like an explosion almost. Because it was so red and white that's what caught me, and then the red subdued and it got to white..."
[From "Witnessed" by Budd Hopkins (1996) p186]