What happened

Rivian Automotive (RIVN 1.63%) shares have more than doubled in the last three months, but that run might be coming to an end. After recovering from an early drop in yesterday's trading, Rivian stock is heading lower again Wednesday morning. As of 10:25 a.m. ET, Rivian shares were down by 5.3%

So what

News from the competition looks to be what's driving the stock lower today. Ford Motor Company (NYSE:F) released its July sales data today, and electric vehicle (EV) sales dropped by 18% compared to last year. That's especially notable since the automaker has been working hard to ramp up production this year. Ford also sold nearly twice as many hybrid vehicles as EVs last month. Its hybrid sales jumped more than 31% year over year. 

Since Ford is currently Rivian's biggest electric pickup truck competitor with its F-150 Lightning, that report might signal a demand issue for Rivian. That wasn't evident when Rivian reported its second-quarter vehicle production and deliveries one month ago, but investors will want to hear what the company has to say when it provides its full second-quarter update on Aug. 8. 

Now what

Until that report is announced, investors are going on what information they have. And whether demand is waning for electric pickup trucks or not, more competition is definitely coming for Rivian. 

Even with Ford's drop in EV sales, it said yesterday that it has completed retooling work at the plant that produces the F-150 Lightning and has restarted production. The company said the facility will have triple the production capacity by this fall. 

And it's not just Ford that Rivian investors will need to watch. General Motors will be entering the market next year with the electric version of its Silverado pickup truck. Ford's sales slump and the growing competition have investors taking some profits after Rivian's recent stock surge. But the next catalyst should come next week when Rivian reports its quarterly update.