This is not mysterious, this is likely the work of Western Intelligence, as part of their Iran, Pakistan, Syria warmongering. When you see something explained away as being caused by "a nearby military drill", you know you're being lied to, like in Iron Man. The insane war against Iran has already begun, this is a last ditch last ditch effort by the failing Pan-Lenninist Banksters.
Russia is not and will not launch infantry operations in Syria, says senior lawmaker (Reuters)
from the article: Report: Mysterious blast in Iran's Isfahan damaged key nuclear site