I believe that Iran has a large number of personnel inside Gaza and the West Bank. The Palestinians have tunnels dug throughout those territories for moving people and materials. Is it really a stretch to think of them digging tunnels deep into Israel proper? Tunnels that go underneath Israeli military instillations and population centers and are being dug for the sole purpose of planting nukes. Israel is a very small country, and most of the nukes they have ( I have a feeling it's not as many as most people think) are stored on a handful of sites.

Should those sites be destroyed by nuclear explosions, then Israel's ability to respond with most of it's population and military infrastructure gone will be quite limited. Iran might get hit by a dozen or so retaliatory strikes, but I think Iran's leadership would be willing to take the hit.