Trucking company failures are rising as faltering freight demand exposes operators unprepared for a downturn after last year’s red-hot shipping market.

Approximately 640 carriers went out of business in the first half of 2019, up from 175 for the same period last year and more than double the total number of trucker failures in 2018, according to transportation industry data firm Broughton Capital LLC.

This week, Denver-based HVH Transportation Inc. abruptly shut down, stranding about 150 drivers and loads out on the road. The closure adds to a 2019 tally that includes Ohio truckload company Falcon Transport Co. and regional less-than-truckload carriers New England Motor Freight Inc. and LME Inc.

Former HVH Chief Executive John Kenneally said he is negotiating with the carrier’s bank on steps to help get the drivers’ fuel cards reactivated so they can deliver their loads and get home. The company has about 380 trucks, including those tied to a related Canadian company, FTI Transportation, which also shut this week, Mr. Kenneally said. That company also belonged to HVH’s owner, private-equity firm HCI Equity Partners.

The increasing number of closures come as trucking companies that boosted driver pay and plowed last year’s profits into record orders for new equipment now are wrestling with a tougher pricing environment and slackening demand.

In 2018, “demand was so strong, rates were so strong, it was virtually impossible to fail,” said Donald Broughton, Broughton Capital’s managing partner.

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Trucking rates stalled out in July, falling 0.1% from the prior year after a 27-month run of annual increases, according to the Cass Truckload Linehaul Index, which measures per-mile pricing for truckload carriers. Prices on trucking’s spot market, where shippers book last-minute transportation, were down nearly 19% last month compared with 2018, according to online freight marketplace DAT Solutions LLC.

That swing has hurt transport operators that shifted more business to the spot market last year to take advantage of surging rates. That also has proved painful for smaller operators that depend more on trucking’s spot market and may not have the leverage big trucking companies have with shipping customers to build higher prices into their contract rates.

Some trucking companies say rising insurance costs also are weighing on the business. Mr. Kenneally said HVH’s monthly insurance bill more than doubled this year, to about $368,000 from $150,000 in 2018.

Some trucking executives believe the recent spate of smaller carrier bankruptcies will give “larger carriers added control and pricing power in the marketplace,” Cowen & Co. transportation analyst Jason Seidl wrote in a research note last month.

Many big operators both bolstered their balance sheets during last year’s freight surge and deepened their ties to shipping customers with a broader array of services.

This week, Iowa-based truckload operator Heartland Express Inc. bought trucker Millis Transfer Inc. for about $150 million. Heartland last month reported its net profit jumped 25.6% in the second quarter despite declining revenue, and cash on the truckload carrier’s balance sheet jumped more than 27% from the end of 2018 to June 30, to $205.6 million.

Write to Jennifer Smith at jennifer.smith@wsj.com