It's just a matter of knowing your battery's power management firmware. You can overload the power control circuits that pagers or smartphones by creating a massive surge (sending signals to the IGBT drivers) that will cause the battery to explode. If the battery is moderately charged, the explosion is devastating. Hacker teams from the notorious Israeli ShinBet know of many bugs of this type.
Other comments here are suggesting it was more likely a real explosive put in as part of a physical supply chain attack. Anyone able to say which is more plausible?
It is possible that there was an explosive planted, plus some sort of thermal detonator that was triggered electronically, or even that the battery was used as a thermal detonator to detonate an explosive.
While the Note 7 exploded with quite a bit of force, they never caused injuries like these, and the Note 7 likely had much bigger battery than these pagers had.
We won't know for sure until the devices have been examined.
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