I have no real, proven clues as to Sonar V Dolphins; but on several times in the past, I've seen Dolphins in the near distance, off Jersey and Plymouth for a start.
On each occasion when I turned on the depth sounder ( not Sonar ) the animals homed straight in on my boat to formate within 2-3' - have photo's to prove it...
So I'd go along with the theory that Sonar / undersea systems might be the cause, though there are probably other environmental factors as well.