Iran shot a barrage of missiles into Iraq aimed at U.S. targets hours before they shot down their own plane. They created a situation that, in their mind, the U.S. was poised to retaliate. This is what they escalated the crisis to. Prior to their attack on the U.S. targets, no rational person thought the U.S. was going to attack targets inside Iran. But once Iran fired those missiles, they knowingly produced a very real potential of retaliation. They were obviously unequipped to handle that potential.
The U.S. is not to blame in the least. That would be like saying Iran is to blame for Iran Air Flight 655 because a few months prior the U.S. destroyed Iran's Navy for mining the Persian Gulf and the U.S. was worried they were going to retaliate.