I could see how this could happen a number of ways and so I pay careful attention to the steps taken when I tow tubers myself:
This could happen if:
- Tube is at swim platform while boarding and engine is running or
- Tube is boarded at swim platform and engine power is used to move away from it. Dangling legs are still near the transom
- Boat stops short while underway, causing tube to continue moving forward toward the stern of the boat (and propellers do not stop spinning instantly)
- Operator drove too close to downed tuber in the water
- Operator couldn't see downed tuber due to turning to port instead of starboard for retrieval (for starboard helms) which could cause #4 or worse
- Just leaving the motor running while a person is climbing aboard or is in the water near the boat
- Tow line gets caught and wraps around propeller, yanking the tube inward
I'm quite the fatalist and envision worst case scenarios with everything (although my friends consider me a positive person). So, I positively won't have any of these negative things happen on my watch.