When your lane change it needs to be done gradually, not too sharply or take too long. Normally you should be over in 4-6 seconds. You should also end up nicely centered in your lane which is accomplished by looking ahead in the new lane. If you turn too sharply you could lose control or if you made a mistake (such as forgot to check your blind spot) there would not be time for the other car to warn you of their presence (Horn). These would be marked as a major error.