- Euron's ships appear from the mist. It is obvious from Yara's reaction that their presence was unnoticed.
- The initial attack occurs by catapult fire. The counter tactic to catapult fire is to spread yourselves out. If you stick together, the catapult only needs to aim at the center of your group and it is liable to hit someone. It is not a precision instrument, it's unlikely to hit its intended target. Therefore, if the ships spread out (which is the best tactic), this gives Euron the opening to sail in between the spread out fleet.
edit An even better point: Being bombarded by catapult fire would give Yara's ships a reason to want to sail into the mist. This may have been an intentional tactic by Euron.
There are also a few things to take into consideration. Though they are from the books, and the show could have diverged from the books in this regard, it has not yet been confirmed that the show has actively contradicted the books:
- Euron plays with the supernatural, he is a very lovecraftian character. It is possible that he controlled the mist, therefore managing to sneak up on the fleet, who did not consider the mist to be anything more than a natural phenomenon.
- Euron's flagship is called the Silence, as its crew are all mutes. Although I'm not sure how deep this goes in the books, it's possible that the Silence intentionally operates more quietly so as to not give away their position, thus making it capable of sneaking up on the enemy.
If Euron's sneak attack is very unusual, I expect the next episode to elaborate on this. Otherwise, it seems fair to assume that this is a normal (though very clever) tactic.