I'm in a similar boat, being rather uneducated here. But I thought it wasn't completely unusual for countries to prod each other's air space. It seems really dangerous for Iran to just auto fire at an encroaching aircraft, potentially starting off a war. In that sense it may be lucky it wasn't a US fighter.
Apparently not so easily recognizable considering that the Russians shot down a Korean Air flight in 1983, the Americans shot down an Iranian flight in 1988, MH17, and now this.