It's clearly caused by bad weather. Iran didn't want him dead. Israel did but I don't see a scenario where they could have done something.
If there's no reason, there's usually no outside involvement, but in this case there's such reason: a warning.
Iran has recently angered some powerful nations with basically unlimited resources, so I wouldn't be surprised if someone placed a small explosive device on that helicopter and waited for a good moment (bad weather) to detonate it. I'm talking about a micro device, the size that could cut a wire or sever a hydraulic connection. An IED the size of a coin.
If it wasn't an accident the Iranian side will probably want to keep it a secret anyway because they don't want the world to know their security detail had a mole, or failed to protect heads of state.
Possibly be that there could be conspiracies which might either be traceable in time or untraceable. But since it's not mentioned yet that it was a linkage of being a homicide action, let's not intend to fumigate falses but like I said, there're lot that the media are always keeping off from the public.
Sincerely expressed, the scenario doesn't detect ordinarily because there'd be chances to Suspended the flight during when they noticed the bad weather to be a safety barrier if that was the case.
Just you @darkangel11 has said, Iran has been engaged with enemies in both recent and previously which could result to the accidental occurances. Let's also not underestimate the power of governments being mean to achieve their goals. It's usually beneath how we expected it.