"At least eight people were killed and 2,750 others including Hezbollah fighters, medics and Iran's envoy to Beirut were wounded on Tuesday when the pagers they use to communicate exploded across Lebanon, security sources and the Lebanese health minister said...
Lebanese internal security forces said a number of wireless communication devices were detonated across Lebanon, especially in Beirut's southern suburbs, a Hezbollah stronghold. The pagers that detonated were the latest model brought in by Hezbollah in recent months, three security sources said."
Most likely hijacked a shipment and replaced it with bombs.
It seems like this is the answer. The shipment that came in 5 months ago was contaminated with the explosive, which was detonated when the hack turned the temperature of the device up to the ignition point.