Lithium batteries can be made to explode under the right conditions.

There's one of two things going on here:

1. The pagers were perfectly normal off the shelf pagers, maybe with some minor flaw to safety or charging systems but nothing that was intentionally introduced. In this case the zionists found a way to hack the pagers (or potentially cell towers to broadcast something that causes the condition) to induce conditions leading to a battery fire/explosion. I don't know enough about lithium batteries to say whether it's suspicious so many of them exploded instead of catching fire though reports say at least some people noticed them getting strangely hot and discarded them which does suggest lithium battery overload.

2. The zionists knew about their use of pagers and in some way intercepted or replaced shipment of the ones they received with a batch that could be triggered to explode. For plausible deniability I'm guessing it was still a battery explosion rather than micro-explosives but we'll have to see if any more info comes out.

Either one is problematic for Hezbollah's communications though at least 2 can be addressed by attempting to do more secure sourcing (e.g. getting Iran to get a direct shipment through an intermediary from a Chinese firm and securing that to be shipped directly to Lebanon). If it's 1 there's not much they can do other than do an investigation on exactly how it happened and contact the manufacturers and hope one of them responds by offering a fixed model not vulnerable to this technique.