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.
The WSJ report claims that:
> The official said some people felt the pagers heat up and disposed of them before they burst.
Sounds more like a battery explosion, but it's way too early to tell.
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.
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?