Tesla seems to inextricably tie the two together. That's too much innovation at once (ergo, more than double the risk). If he just focussed on making awesome electric cars (with auto-drive as an option), it would 1) make Tesla's job easier, 2) make their PR reps job easier, and 3) everyone is positive about green tech. Winning over the BMW/Merc fans with high performance electric cars seems like a better strategy for a car company than trying to push a fleet of average performing occasionally fumbling self driving cars on the streets - from the perspective of the short sellers.
So in the current scenario, everytime there is some news about how the auto-drive confused one thing for another and crashed into it, the short sellers are convinced that there will be more of a negative public feedback cycle which will bring Tesla, or at least its equity, down. So they short.
Source - I have shorted Tesla in the past after the auto drive crashed and killed a driver. I am also technical enough to be paranoid about trusting human lives to a not-100% reliable piece of tech at high speeds. I only trust our attitude of move-fast-and-break-things with things like websites, not things which could kill you.