Most likely lack of maintenance.  Corruption plays a part.  And Russian involvement in the program.  “Massive fire breaks out near Iran’s Bushehr nuclear power plant – Watch,” Jerusalem Post,” May 8, 2021.

“[A] senior delegation of Israeli officials who met with their White House counterparts (and also with President Biden) concluded after their own talks that Biden would give Iran billions of dollars in sanctions relief without first fixing the nuclear deal’s flaws – which include, in their view, letting Iran proceed with an advanced centrifuge program in 2023 and without enhanced verification of Iran’s compliance with the deal.”  See “Judith Miller: Is Iran’s Supreme Leader near death? Why the answer is supremely important,” Judith Miller, Fox, May 8, 2021.

Notice the theme of the following three stories.  Destabilization.  Doesn’t happen by accident.  “Iran, Hamas, Hezbollah join to incite terror against Israel,” Seth Frantzman, op-ed, Jerusalem Post, May 8, 2021.

“Morocco Says Iran Working on Destabilizing North, West Africa,” Asharq Al-Awsat, May 8, 2021.

“Iran-backed PMFs are destabilising Iraq’s disputed regions,” Kamaran Palani, op-ed, Al Jazeera, May 8, 2021.