I immediate thought that, in part, the dolphins might be attracted by, intrigued by the cat's probably purring, sensing it through their jaws. But there seems to be a patience and curiosity there that speaks to significantly more than mere "animal instinct".
We already communicate with dolphins, and experiments have shown that they can understand the syntax and semantics of request/command structures (the same communication elements in different orders ('sentence structures', so to speak) have different meanings).
What will they say when we finally understand them? "Took you long enough."