Considering this happened a few hours after Iran launched it's fireworks at the US, I imagine every air defense unit in Iran was on the highest alert, with very itchy trigger fingers.
It doesn't seem that far fetched to imagine terrified Iranian air-defense personnel, making a big mistake, under pressure of being attacked by the biggest air force in the world.
After all all they see is a blimp on a radar screen, they can't tell if it's a 737 or tornado or can they, anybody worked on radar?