Two reasons that are tied together at the hip:
1) It's a pretty strong pattern that people use to trade. Doesn't mean it happens every time, but it's common enough for people to use across many companies.
2) The way I think about it, IV builds due to the uncertainty around each earnings announcement. Is it a good one? A great one? A bad one? A really really bad one? True for any company, not just TSLA. So IV ratchets up as people take each side (one of the trading strategies is a straddle where you take BOTH sides
).
Once the earnings announcement happens, you have resolution of the uncertainty. That resolution drops the IV significantly (which changes the option premium by Vega). At least that's the theory I'm working from.