Unclear from reporting how they were triggered. Some possibilities include:
- completely offline (all the compromised pagers were pre-programmed to beep and explode at a particular time)
- a broadcast signal (possibly sent by a high power transmitter controlled by Israel) that all the devices were programed to respond to
- individually addressed messages to each of the pagers (less likely, since that would take a while to go through).
My guess is the first.