They probably launched a virus (a la Pegasus) onto the pagers to cause the batteries to overheat. Israel is known for daring covert operations, but even for that country this attack would be exceptional.

PETN is a secondary explosive, meaning it is more difficult to detonate than primary explosives. I only know it was/is a major part of Semtex.It is a very powerful explosive material first prepared and patented in 1894 by the explosives manufacturer Rheinisch-Westfälische Sprengstoff A.G. [de] of Cologne, Germany.