What happened 

Shares of electric-truck makers Workhorse Group (WKHS 9.97%) and Nikola (NKLA -0.34%) were both trading lower on Wednesday after a longtime investor in Tesla (TSLA 3.17%) said that it had reduced its stake in the electric-vehicle giant. 

As of 2:30 p.m. EDT, Workhorse's shares were down about 6.7%, and Nikola's down about 4.1%, from Tuesday's closing prices.

So what

Tesla's shares fell sharply on Wednesday morning after investment management firm Baillie Gifford, the largest institutional Tesla shareholder and a longtime supporter of the company, said that it had reduced its stake in the company from about 6.3% to below 5%.

A silver Nikola Badger, a brawny electric pickup truck.

Nikola is expected to announce a manufacturing partner for its Badger pickup truck soon. But what if the electric-vehicle bubble pops before then? Image source: Nikola.

In a statement, Baillie Gifford fund manager James Anderson said that the move was for portfolio-management reasons (after Tesla's huge gains in 2020, Baillie Gifford's stake had grown too big) rather than because of a change of heart about the company. 

Still, investors seemed concerned about the news. Tesla's shares were down over 14% early on Wednesday before recovering some ground. (Tesla's stock was down about 6.7% from Tuesday's close as of 2:30 p.m. EDT.) 

It was likely those concerns dragged down the stock prices of several more speculative electric-vehicle companies, including both Workhorse and Nikola. 

Now what

Given that Tesla's stock price has increased roughly tenfold over the last year, it's no surprise to see some selling pressure surface. Even longtime Tesla bulls (like my Foolish colleague Daniel Sparks) have conceded that the company's current valuation is increasingly hard to justify. 

The recent share-price gains for both Workhorse and Nikola are due at least in part to auto investors' excitement following Tesla's impressive run. But those gains have always been at risk: If Tesla's run falters, or its stock price falls sharply, that investor excitement -- and the prices of Nikola, Workhorse, and other electric-vehicle stocks -- could fall as well. 

While both Workhorse and Nikola have promising growth plans, the risks in the near term are significant. Invest accordingly.