There are of course at least two possible explanations: the market either thinks the SCTY merger is less likely than it did last week or the shorts are taking no chances of a run up in stock price of SCTY after Friday's unveiling of gorgeous solar roofs and is sending a false signal designed to protect the current (and very substantial short positions held by many in SCTY). I vote for the second explanation.
When Tesla announced plans to acquire SCTY on June 21, big SCTY shorts lost millions on that announcement. There's now a large number of shorts with malice in both SCTY and TSLA who believe that if they can hold tight, TSLA and SCTY will remain low. They hope that maybe the SCTY merger will go away or that TSLA will be unable to do an equity raise and will run out of money prior to Model 3 ramp-up if they hold tight and significantly affect the stock price.
For investors, the stock looks sick, but much of what we're seeing is shorts doubling down as more good news comes forth. The stock will continue to look sick until it's breakout. Once the SCTY merger goes through, the short positions in SCTY get transferred to TSLA, I believe. Once the logjam price of $214 or $215 is broken at TSLA, the blockade by the shorts falls apart, panic buying ensues, and we see a squeeze as likely. The event I think most likely to bring about such a rise in TSLA is the 4Q ER. Demand is there, production speed is there, and all we need is no surprises and no horrific winter weather in 2nd half of December to delay deliveries.
Can the shorts win in the short-to-mid-term? Yes, if there's a large, unexpected macro event. Otherwise, I think the chess board favors the longs.