TX, its because, at the time of the tragedy. There were reports coming in from every person who was within speaking distance of a reporter.
None of them made any sense, and very few of them were in any way factual. Take for instance, the report early in the day that there were two shooters involved. This was because a local had heard what was happening, and was on his way to assist. The police had arrived before the local did and not knowing themselves what exactly was going on. They detained the man.
Second, there were reports that there was a long gun found in the car. Many of the "reporters", and I use that term VERY loosely, ran with it being a "machine gun". They didn't bother to check any facts. For the simple reason that no one person had any tangible facts to give.
We will likely not know exactly what happened that day. What we will know is that too many people gave too many false reports. In an effort to be the first one with a lead to tell the masses.