Shares of GoPro (NASDAQ:GPRO) have crashed more than 80% over the past six months because of botched product launches, slowing sales, and concerns about market commoditization. However, two other stocks have been dragged down with GoPro -- its chip supplier Ambarella (NASDAQ:AMBA) and fitness tracker maker Fitbit (NYSE:FIT).

Image source: GoPro.

Ambarella has plunged 70% over the past half year, while Fitbit has fallen 60%. On Jan. 14, the day after GoPro warned that its fourth-quarter sales would fall 31% annually, Ambarella stock tumbled as much as 14%, and Fitbit fell nearly 9%. Those lockstep declines raised an interesting question: Are investors overreacting by dumping both stocks on GoPro's woes?

A closer look at Ambarella
Ambarella supplies image application processors for action cams, drones, dash cams, security cams, and other connected devices. Ambarella doesn't break down its revenues by category, but Pacific Crest analysts estimate that GoPro orders will still account for as much as 25% of Ambarella's sales this year.

Last quarter, Ambarella warned that its fourth-quarter sales will only rise 0.5% to 4.3% annually, down from 61.9% growth a year earlier and missing the consensus estimate for 17.9% growth. The chipmaker attributed that slowdown to "near-term headwinds" in the action camera market. Ambarella also supplies chips to GoPro's rivals, but CEO Fermi Wang noted that most of those competitors remained "far away from GoPro's numbers" during last quarter's conference call. Analysts now expect Ambarella's sales to rise 44% in fiscal 2016 (ending on Jan. 31, 2015), and just 13% in fiscal 2017.

Image source: Ambarella.

Ambarella bulls often highlight the company's growth potential in drones. Unfortunately, image processing chips for drones still only account for about 10% of Ambarella's sales. Meanwhile, Qualcomm (NASDAQ:QCOM) has been aggressively expanding into the same market with mobile SoCs for drones. Last June, Morgan Stanley analysts declared that Ambarella's SoCs had a cost advantage against Qualcomm's SoCs, but that it would lose that advantage if cellular connections became standard for drones.

A closer look at Fitbit
While Ambarella's connections with GoPro are obvious, Fitbit's relationship with the action camera maker is less defined. The two companies sell different products, but investors are worried that their sales are following a similar trajectory.

GoPro's sales rose 87% in 2013 and 41% in 2014, but the company expects sales growth to slow to just 15% for 2015. In 2016, analysts expect that figure to plunge to 0.6% thanks to the commoditization of the action camera market. Fitbit's sales are losing momentum in the same way. Revenue rose 255% in 2013 and 178% in 2014, but analysts expect growth to slow to 144% in 2015 and just 33% in 2016. But if Fitbit's earnings and guidance miss expectations in the fourth quarter, that sales forecast could be too high.

Image source: Fitbit.

Fitbit and GoPro both face two identical problems. First, it's easy for competitors to produce similar products with cheaper price tags. That's why Xiaomi targeted both Fitbit and GoPro with its $15 Mi Band and $90 Yi Action Camera. It's unclear how much Xiaomi is hurting GoPro in China, but IDC claims Xiaomi's share of the global wearables market surged from 5.7% to 17.4% between the third quarters of 2014 and 2015. During that period, Fitbit's share fell from 32.8% to 22.2%. Second, GoPro cameras and Fitbit trackers could be respectively rendered obsolete by better cameras and motion-tracking chips in new smartphones.

Is one of these stocks not like the others?
It's silly to blindly sell off all three stocks together, but it's also reckless to ignore the connections between the companies. Looking ahead, GoPro and Ambarella both believe rising demand for drones will diversify their businesses away from action cameras.

Unfortunately, there's no guarantee drones will evolve from niche devices into mainstream ones. Moreover, their high price tags and new registration rules in the U.S. could impact sales. So, unless Ambarella clearly discloses its sales figures from GoPro and shows that it can survive without action cameras, its stock will remain tightly chained to GoPro's.

Fitbit's situation isn't as dire as GoPro's yet, since it might carve out a niche market in sports performance devices, or generate higher sales through corporate wellness programs. But until Fitbit can prove that its sales won't hit a brick wall like GoPro's, its stock will continue selling off on negative GoPro headlines.