If the footage on Twitter is legit, then there was a small detonation, with a bang, not a burning battery with a 'whoosh' and flames. Which indicates that the internals of the pager had been replaced with something rather more explosive than a lithium battery.
Lithium ion batteries in devices are sandwiched layers enclosed in a kind of 'pouch', right? So what if you manufactured one of these that looked identical to the normal battery, but only had half a battery inside, and the rest of it was plastic explosive. Maybe put a tiny chip in there that, when a particular pattern of current draw happens, fires a detonator. Then, some firmware hack in the device proper that responds to some event and actuates that current draw pattern. It wouldn't even look suspicious if you opened it up.
That's an interesting idea, and it wouldn't even need a firmware hack... a real time clock circuit with a specific date/time to detonate would be simpler and easier to coordinate simultaneous detonation.
