[Unclenick]

Federal has long had a reputation for having the most sensitive primers, and the rumor was that a Lee customer had one go off that caused sympathetic ignition of all the others in the tray, resulting in a law suit. That large a number of primers going off together could be a very serious hazard. So, if the rumor is correct, that's when Lee decided they didn't want anyone else risking it, and hence the flame.

More recently though, if you read reviews on Midway and a couple of othe places, you find Winchester primers are the ones that get hammered for piercing and other problems. Sometime around 2,000 (late 90's, I think, but I don't trust my memory on that point), Winchester decided to address complaints about inadequate sensitivity by ceasing to nickel plate their primer cups. Seems like they accomplished their goal: softer cups.

We had a photo in a post on the board awhile back of a carton of Winchester primers that had partially detonated in the box during shipping. Don't know if it was dropped, or what, but it didn't look good.

Another thing that can set primers off is priming mix dust building up in a priming tool. Dillons have had very occasional ignitions in their reinforced feed tubes due to this that have put quite a pattern on the ceiling above them. Only tiny bits of the dust leak out at a time, but eventually it accumulates somewhere, and if it got pushed up into the primer cup, it could go off and light up the rest of the primer. Anyway, the bottom line there is to disassemble and clean primer handling equipment with alcohol periodically.

Also, wearing hearing as well as eye protection is often advised, for reasons described in the OP!