The sensation of falling in sleep is called hypnic jerks.
Almost, 70% of people experience hypnic jerks.
The stage at which jerks happen is known as hypnagogic stage.
This happens due to involuntary muscle jerks or loosening of muscles.
Hypnic jerks are totally normal as its a way of brain reacting to muscle loosening up. The brain mistakes it to be a sign of falling and tenses and tries to prepare the body to stop the fall.