As reported in the Jerusalem Post, from an interview with (you guessed it) Fox News. The reason? Russia, which has promised the Iranians the fuel for the reactor, will begin to load that fuel in eight days- after which point, any airstrike would likely disperse deadly radioactive material that would kill or sicken many, many people. This was discussed a few days ago at some length by Marsha Cohen at Asia Times, who noted then that this deadline might fuel speculation about an imminent strike. She notes further that the Iranians had run an air-defense drill (which caught locals by surprise) over Bushehr. And, as she also notes, there's nothing whatsoever illegal about Iran's possession of a working nuclear reactor:
The prospect of Bushehr becoming operational coincides with the proliferation of public statements that claim an attack on Iran by Israel or the US is impending and inevitable. Bushehr is strategically located in southwestern Iran on the Gulf coast, directly across from Kuwait.
An aerial assault on Bushehr would have to take place before any nuclear fuel arrives at the site. Beyond that point, an attack on the reactor would release deadly radioactive fallout into the entire Persian Gulf region and beyond. Besides the catastrophic human and environmental toll of such an attack, the sea lanes through which much of the world's oil supplies pass would be endangered.
The Iranians know this. In 1980, Iran bombed Iraq's Osirak nuclear power plant before it contained any radioactive material. Osirak was quickly repaired by the French contractors who built it. Eight months later, Osirak was partially destroyed by Israeli jets, aided by Iranian intelligence.
Nothing about Bushehr violates any international agreements to which Iran is a party. The International Atomic Energy Agency (IAEA) was created to promote the use of "atoms for peace". The nuclear Non-Proliferation Treaty (NPT), which Iran signed in 1968 and ratified two years later, obligates the five nuclear-weapon states (the US, Britain, France, Russia and China) to assist non-nuclear weapon states that signed the NPT in acquiring and utilizing nuclear technology for energy production and other peaceful purposes.
Under the NPT, Iran has the right to produce its own nuclear fuel for civilian projects such as Bushehr. However, suspicions have been raised for nearly two decades that Iran might try to convert low enriched uranium for electricity generation into highly enriched uranium.
The IAEA's approval of Iran's nuclear energy program is contingent on Iran buying its fuel from approved suppliers abroad, and exporting its nuclear waste back to its source so that the radioactive material it contains can't be diverted for use in weapons of mass destruction or fall into the wrong hands. Russia qualifies as an approved supplier.
Sergei Kiriyenko, the head of the Russian Federal Atomic Energy Agency (Rosatom), also told the Russian news wire service Interfax on July 27 that Bushehr would not be affected by United Nations Security Council sanctions against Iran.
He said, "No one is against the development of Iran's civilian nuclear program; the construction of the Bushehr nuclear power plant is being carried out under the control of the International Atomic Energy Agency."
Russia has guaranteed that it will supply all the nuclear fuel needed by Bushehr, and that its nuclear waste will be reprocessed in Russia.
Israeli military and politicians usually equate Iranian access to nuclear fuel for electrical generation with Iran's acquisition of a nuclear weapon. A light water reactor, Bushehr won't be capable of producing weapons-grade plutonium (unlike Israel's heavy water reactor at Dimona).
However, Bushehr's becoming operational would affirm Iran's right to develop and utilize nuclear technology, and give Iran the status and prestige of a nuclear power. Israelis claim this would pose an "existential threat" to the Jewish state.
Once Bushehr's nuclear fuel arrives from Russia, whatever military options against Iran that may be "on the table" that include Bushehr will have to come off. Israel and the US have only a few weeks to launch an attack on Iran before Bushehr has the means to begin generating electricity.
Israeli sources have often hinted that a strike against Iran might be conducted with precision-guided drones, in order to minimize casualties among Israeli soldiers. It's a possibility for which Iranians feel they need to prepare, which may explain the report of drones over Bushehr as the nuclear facility prepares to come online.