Undercover agents, double agents, or someone disaffected by Hezbollah could have replaced the devices gradually. Another possibility could have been sabotage during routine maintenance. This operation may have been underway for more than a year or two.
I think the secondary objective of this wave of explosions was to cut off enemy communications. But the side effect was to create fear and confusion among the civilian population. I imagine people must be terrified of any device such as radios, cell phones and even notebooks.
