I heard this second or third hand from somebody who worked in the tabernacle before it burned down. Apparently BYU had brought in a professional lighting contractor for the concert, because they were switching out some of the lights. This person noticed that one of the contractors was taking the old lights out of the ceiling, and sticking them up into the attic without disconnecting the power first, and then filling the holes with the new lights. He warned the guy that if somebody turned the old lights on, they would start a fire in the attic, but the guy assured him that nobody was going to turn the old lights on. So they were left connected, lying on the bare wood, or whatever insulation was in the attic. Of course, it would be hard to find out what actually happened, but it's not hard to imagine how somebody could have turned the all the lights on (including the old ones) for the rehearsal, they smoked and smoldered in the attic and people reported strange smells, then when the rehearsal was done, they turned off all the lights they could see, leaving the ones in the attic on. The smoke triggered the previously faulty alarms, which get ignored, and a few hours later, you get a fire that mysteriously starts in the attic.
