Unless the plastic case is strong, a runaway Li-Po battery probably wont explode with a lot of force.
Most have a protective circuit right at the battery to prevent overload and excessive temperature.
This is not part of the pager, phone, laptop but part of the battery.
This could have been modified.
Where Li-ion or Li-Po batteries are most dangerous is Hard cases that can contain pressure until rupture OR
where the battery is physically compromised, like a puncture shorting many layers at once. Most Li-Po cells have a foil outer pouch that will swell, popping a case apart, burst and maybe explode from overheating. Puncture all layers and it's much more violent.
It would take a small charge to drive a conductive pin or edge into the battery to cause rapid deconbobulation (maybe after a short thermal effect).
Puncture and explosive gases are rapidly released.
The pager software/firmware could be remotely modified to trigger the charge with a predetermined signal but installing the charge would require actual physical access to each device.
Since pagers send out individual signals to each unit it would take some time to send individual pages to a couple thousand units.
Reports say the explosions happened over the course of an hour or so.
That indicates individual addressing.
You can find second hand units of this model on ebay.