Rocket Lab USA, Inc. (RKLB) shares tumbled 5.03% in after-hours trading on Monday, following the release of its second-quarter earnings report that featured a weaker-than-expected gross margin forecast for the third quarter.

The sell-off was triggered by the company's projection that Q3 GAAP gross margins would land between 29% and 31%, well below analysts' average estimate of 37.6%. Rocket Lab attributed the softer margin outlook to a shifting product mix, with sales of lower-margin satellite platforms expected to make up a larger share of its business going forward. The Long Beach, California-based company is increasingly booking more revenue from building larger, more complex satellite platforms, which generate lower margins than some of its other space hardware and launch products.

The mixed results also contributed to the decline. While Rocket Lab posted record second-quarter revenue of $234.1 million, up 62% year-over-year and beating estimates of $231.4 million, its loss per share of $0.08 missed the consensus estimate of a $0.07 loss. On the positive side, the company guided for Q3 revenue of $250 million to $265 million, above Wall Street's $238.5 million estimate, and ended the quarter with a record backlog of $2.36 billion, up 137% year-over-year.