I'm the U.S. Navy, the Enlisted Submarine Warfare pin is known as "the dolphins." It has a sub with bottlenose dolphins on either side. Dolphins hunt in packs, and will often run in giant groups when chasing tuna and other fast meals, like that tweet.

I don't think he's referring to subs, though. I think he's referring to "the hunters become the hunted" with this tweet. Dolphins will just run down orey and wear them down, then group them all together, and swarm in for the easy kill.