My headcanon is that Elon downloaded Blender one night to try and learn it, made a low-poly car and then some sycophants said "hey great job" and he decided it was now cool and Tesla should manufacture it.
Mine is that he was watching Back to the Future after moving to Texas and thought: "But what if it was a truck!"
The talk about the hinges starts at 18:45[1] in the video but I recommend you watch the whole video from the start.