Comment by blantonl

Pagers just simply have an address (called a Cap Code) to receive messages. It's like a mailbox number. A pager can be programmed with usually up to 4 Cap Codes at a time.

If I was speculating on what happened, I would bet that the pager had 3 Cap Code addresses programmed, the mailbox cap code the owner of the pager expected to have for receiving messages, a cap code that was the same programmed in all the pagers to that functioned normally to received messages, and then the 3rd cap code programmed in all the pagers that when receiving a specific message triggered the explosive.

The folks responsible simply sent a message to the 2nd cap code to get all the pagers to go off, presumably to get the targets to get the pagers out and look at them, and then immediately the trigger message next to the 3rd cap code to detonate the explosive.