premise: CAIRO â€” Egypt's Interior Ministry says a man was killed and three members of his family were injured when a device exploded while he was cleaning up his backyard in a leafy Cairo suburb. The police didn't elaborate on whether the device is a bomb and they could not specify what exactly triggered the blast on Friday. The ministry says the blast occurred in the southern residential suburb of Maadi that is also home to many diplomatic residences in the Egyptian capital. The statement added that security forces have cordoned off the area. No further details were immediately available.
hypothesis: The bomb was set off by an animal.
reason: It isn't known how the bomb was triggered.