Yes, I did know why. It's pretty easy to put 2 and 2 together when explosives and radios are together like that. The radio could set off the explosives. I also asked my 9 year old brother about. (He was building crystal radios at the time) There was quite the discussion, as we traveled in a pickup truck/camper combo with the kids in the back. We used an intercom between the camper and the cab of the truck. It was wired, but my mom was still freaking out.
But really, all that was needed to be known was that a radio could set off the explosives. Historically, it was known that a two way radio could induce a current in the wire leading to the blasting caps. Later, when explosive charges were set off with radio signals, there was concern that radios (and cell phones) could trigger them, but this (especially triggering via cell phone use) is not nearly as likely as most think.
Cooper himself didn't say why his "bomb" would explode, either, aside from "electronic detonators," which really doesn't mean anything. It certainly doesn't mean he had any explosives knowledge, that he had any of these "electronic detonators," nor why he would need more than one for a bomb that fit inside of an attache case.
Telling the crew to use the radio "as little as possible" is hogwash. If the radio is likely, or even possibly able to set off the bomb you built, then you tell them not to use the radio at all. Or build a bomb that isn't likely to be set off by something you know is routinely used by what you are hijacking. Doing otherwise sounds far from a explosives "expert" to me. It sounds like someone bluffing while trying to put the crew on edge in order to gain a psychological advantage.
