Elon Musk's army of small, retail shareholders has helped prop up Tesla stock in the past. Many of those same retail shareholders now hold SpaceX.

Retail investors, however, have started selling SpaceX stock. The reason why is hard to suss out.

JPMorgan analyst Arun Jain tracks retail flows into and out of stocks. SpaceX saw massive inflows in its first few weeks as a public company. That makes sense. Smaller retail investors couldn't access SpaceX ahead of its record-setting IPO.

Flows have reversed, though. Jain calculates some $570 million in retail selling over the past three weeks. Retail investors took almost $250 million off the table this past week-the largest weekly decline he's seen.

Why the selling is a good question to ask. It isn't stock performance. SpaceX stock is actually up about $20 since the selling began. Of course, retail investors could be taking short-term profits. It could just be normalization of recent trends. Retail shareholders bought a lot of stock early on. Recent selling still leaves retail SpaceX inflows of about $3.4 billion.

That's a small portion of overall SpaceX stock traded. Still, investors should follow retail flows. Retail investors hold roughly 40% of Tesla stock available to trade, according to Bloomberg, which is about twice the level of other large tech stocks.

Tesla's retail shareholders have proven very reliable backers of Musk and his vision. Exactly how that translates into stock performance is tough to say, but it is part of the reason Tesla stock trades for a lofty 174 times earnings estimated over the next 12 months.

That's actually a premium to SpaceX. Elon Musk's rocket and AI company now trades for about 138 times earnings. (Earnings estimates have risen dramatically since SpaceX's second-quarter earnings report.)

SpaceX stock was up 0.8% in premarket trading at $156, while S&P 500 and Dow Jones Industrial Average futures were 0.3% and 0.2% higher, respectively.

Shares were rising after the company was awarded the launch services contract for NASA's StarBurst gamma ray detector. SpaceX will take a "small satellite designed to investigate neutron star mergers and the origins of short gamma-ray bursts" into space "no earlier than 2028," according to a news release.

It's a nice win. It isn't a big surprise. SpaceX remains the dominant provider of launch services in the U.S.