Dick's Sporting Goods shares cratered as much as 30% on Tuesday, the retailer's worst single-day slide in years, after a second-quarter earnings miss and a brutal cut to its full-year outlook laid bare just how badly its newly acquired Foot Locker chain is struggling.

The sporting-goods giant reported adjusted earnings of $3.53 a share for the quarter, missing Wall Street's $3.78 estimate, on revenue of $5.59 billion versus the $5.65 billion analysts expected, according to the company's earnings report covered by CNBC. Total revenue was up 53.2% year over year, but nearly all of that growth came from folding in Foot Locker, which Dick's bought for roughly $2.4 billion in a deal that closed last September.

The bigger shock came in guidance. Dick's slashed its full-year adjusted EPS outlook to a range of $11.00 to $12.00, well below the $14.20 Wall Street had penciled in — a roughly 19% shortfall at the midpoint. Dick's own banner business actually grew, with comparable sales up 4.9%, but Foot Locker's proforma comparable sales fell 3.6%, dragging the combined forecast down, per WWD's earnings breakdown.

Foot Locker's footwear problem

Executive Chairman Ed Stack told analysts the athletic footwear market turned unexpectedly promotional as the quarter wore on, and that Foot Locker's heavy reliance on sneaker "launch" culture backfired when new releases underperformed.

"Not only were there fewer launches in the second quarter, but those launches performed below both industry and our expectations," Stack said.

He added that a wave of brand-driven discounting "spilled into the broader marketplace," squeezing margins on legacy and retro sneaker styles that make up a large share of Foot Locker's business — exposure that Dick's own stores don't carry to the same degree.

The selloff didn't stay contained to Dick's. Nike, Lululemon and On Holding all slid in Tuesday trading on what analysts described as "read-across" selling, as investors concluded Dick's promotional-environment warning applied to the athletic wear sector broadly, per 24/7 Wall St's market roundup. None of those companies had reported news of their own.

The result left Dick's stock at its lowest level in more than two years and, by some measures, on pace for its worst single-session percentage loss on record, a stretch that has now seen the stock fall in 10 of its last 11 trading sessions, according to Motley Fool's market analysis.

The results mark the first full quarter in which Foot Locker's performance has been visible inside Dick's consolidated numbers, and they raise pointed questions about the price Dick's paid to become the dominant player in mall-based sneaker retail just as the category turns more competitive. Management gave no indication the promotional pressure would ease before the holiday shopping season, leaving investors to wait for the next quarterly update — and for signs of whether the Foot Locker turnaround Dick's promised when it announced the deal is still on track.