There are videos capturing lots of explosions going off simulataneously. Since pagers already can recieve messages and these devices were deeply infiltrated, they likely added a special trigger message to set them off. THis could also allow other scenarios, like only setting off one (for whatever reason).
If they can plant explosives they can 100% tap into the circuit or maybe reprogram the board. The CIA is known to be able to do this to specific devices by plucking them from the mail. Mossad could likely do their own production run and ship that to Hezbollah.