Getting batteries to release energy isn't very difficult, even getting them to release it quickly isn't very difficult. What's difficult is getting them to release it over the course of a few milliseconds. Which is what you would need for an explosion.

If the battery simply dumped all its power over the course of 30 seconds that's basically just a fire that you can run away from.

Also I wouldn't have thought a pager had that much charge, I wouldn't have thought this sort of thing would be possible as they would tend to just go off with a loud bang, assuming you could even get them to release all the energy at once l, which again I wouldn't have thought was possible.

For fairly obvious reasons I don't think we're ever going to find out how this was done.