It could be cavitiation you are seeing. Did you make a sharp turn in the area that this happened?
I also see that there is an almost constant line on each side of the centerline. This indicates that something is reflecting back sonar from a close distance. This is either something on your boat or the turbulent water that flows off the hull. Easy way to figure our which is to stop shut the motor off and let the boat drift. If the lines are still there tan it is something on the boat that is causing it. If it goes away than it was the turbulent water causing it.
