While Israel would be the #1 suspect as they want the JCPOA deal to fail, I would assume that some people in Iran are also against the deal and want it to see it fail, so an internal sabotage isn't really out of the question.
I would suspect that most Iranians, regardless of if they are for or against the nuclear program, just want sanction relief and don’t care how they get it. At the end of the day, most people just want to have a good life and sanction relief will help that happen.
Self defence isn't aggression. If you provide a credible threat against me, you better believe I'll punch you in the face and knock you out first.
Since the alignment of the gulf states with the US, the only obstacle to that is Iran and Iranian influence.
Because of this, Israel is very willing for Iran to be militarily destroyed, and members of the Israeli government have called for the invasion of Iran multiple times.
Simply self defence is far from the goal. Israel would not be able to survive without changing its internal structure unless the Arab world is dominated by the US which prevents actions that go against Israeli interests.
Whilst you would expect most airgapped systems to also be in buildings that intentionally block signals, there are a ton of "unexpected" interactions, some of which may give you ways to remotely transmit when no transmission is expected to be possible.
Like the fact that USB 3.0 can interfere with the 2.4Ghz spectrum [0], thanks to resonance.
Israel has also previously demonstrated malware called Odini [1] and aIR-Jumper [2] to bypass Faraday protections around systems and exfiltrate data.
[0] https://www.intel.com/content/www/us/en/products/docs/io/uni...
[1] https://www.rambus.com/blogs/bypassing-air-gapped-faraday-ca...
[2] https://arstechnica.com/information-technology/2017/09/attac...
So it does appear to be someone at the facility.