Mainly, the move is part of the US's attempts in combating North Korea's maritime sanctions evasion activity.
As USCG reports, ship-to-ship transfers of fuels and goods from and to North Korea are prohibited under UNSCRs.
Moreover, in April 2018, the United Nations’ Security Council blacklisted 27 ships as part of its clamp down on North Korea over its nuclear program.
The sanctions consisted of one individual and 22 companies involved in trading of coal, refined petroleum and oil with North Korean counterparts, which had been declared illegal.
Two months after, in February 2018, the US proceeded to imposing sanctions on 28 ships, as well as 27 entities and one individual.
Steven Mnuchin, U.S. Treasury Secretary commented
Our actions are part of the ongoing maximum economic pressure campaign to cut off sources of revenue that this regime derives from U.N. and U.S. prohibitive trade to fund its nuclear and ballistic missile programs.