With a labor contract set to expire for West Coast dockworkers, retailers aren't taking any chances for key delivery of their back-to-school and holiday shipments.
Those are the two top-selling periods in the retail industry calendar.
According to the monthly Global Port Tracker report, released Friday by the National Retail Federation and Hackett Associates, May import volume rose 5.8 compared with the prior year, while June is expected to see a 7.5 percent increase from 2013.
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NRF noted that these are both "unusually high numbers not normally seen until later in the summer or fall."
"[It's] a sign that retailers have begun bringing imported merchandise into the country early because of the uncertainty of what could happen when the labor contract expires."
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Negotiations between the Pacific Maritime Association, which represents workers who operate port terminals and shipping lines, and the International Longshore and Warehouse Union, which represents dockworkers, started last month. The contract expires June 30.
Britt Beemer, founder of America's Research Group, said last week that it's particularly important for retailers to get their orders in early as the holiday shopping season shifts earlier, thus causing retailers to battle for a share of consumers' wallets sooner.
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According to Beemer's research, shoppers who enter a particular store for its Black Friday sales have a 70 percent chance of returning two or three more times that season to buy more goods. If they don't, there's a 30 percent chance they won't come at all.
The last major shutdown for the West Coast ports happened in 2002, when they were shuttered for 10 days.
—By CNBC's Krystina Gustafson