The cable of the dipping sonar isn't sinking as fast as it is being deployed by the helo, so it curls under the helo in the water, twisting the direction of the hydrophone relative to the helo.
So, yes, it is a bug.
Depending on how serious a DW player you are, you might want to check out LWAMI, a mod I had a hand in creating. It fixes this and many other issues that members of the community have had with DW over the year or so of its release on the internet through Battlefront.com, the first publisher.
You can find the readme at this thread:
http://www.subsim.com/phpBB/viewtopic.php?t=49705 .