Considering the larger forces at work in the Justice League universe and… well, Ocean Master may have his reasons to attack in order to protect the kingdom left to him by his half-brother.
It worked (and then some), with Johns pitting Arthur against his half-brother Orm in an all-out war with the surface world and Justice League.
The story called upon the same key players now in place in the DCEU – Aquaman, Mera, Vulko, and Ocean Master – so the potential is present for James Wan to craft a “swashbuckling adventure” on a blockbuster scale – and maybe even pull a Justice League member or two in for a cameo, should the story stay true to the comics.