Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

A combination of troubling earnings and economic data that stoked fears of the Federal Reserve ending its bond-buying program sooner rather than later sent the stock market plunging today, as the market broke out of its lackadaisical trading pattern and showed just how volatile a vacation-ridden August trading session can be. The Dow Jones Industrials (DJINDICES:^DJI) lost 225 points on the day, while the U.S. dollar fell, gold and oil prices rose, and bond yields soared to their highest levels since 2011.

The biggest loser in the Dow was Cisco Systems (NASDAQ:CSCO), which nosedived more than 7% after releasing earnings last night. On its face, Cisco's report seemed solid, with adjusted earnings jumping more than 10% on a 6.2% rise in revenue. Yet massive layoffs of 4,000 workers -- about 5% of its total workforce -- and lackluster guidance on sales and earnings growth were enough to make shareholders pessimistic about the company's ability to keep pace in the cutthroat industry.

Hewlett-Packard (NYSE:HPQ) also joined the tech bloodbath, falling 4.5%. HP will report earnings next week, and with rival Lenovo having reported a 23% gain in profits on the back of its strength in the smartphone and tablet segment, HP investors have to feel nervous about whether the pace of Hewlett's turnaround efforts will be fast enough to avoid getting left behind by Lenovo and other faster-moving competitors. So far, investors have given CEO Meg Whitman considerable latitude to take an extended period of time to implement her policy initiatives, but, at some point, patience could wear thin.

Finally, Home Depot (NYSE:HD) dropped 3% on concerns that the company could see growth slow due to soaring interest rates. The fact that rival Lowe's (NYSE:LOW) apparently won its $205 million bid for bankrupt hardware-store retailer Orchard Supply and its 72 stores probably didn't help Home Depot's prospects, either, although Lowe's fell 2.5% as well. The potential for greater competition in the key California market could threaten Home Depot's business there a bit, but more troubling would be any macroeconomic headwind that hurts housing's fragile recovery.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.