A dynamic rollover is a catastrophic event where rotor thrust pulls the helicopter sideways around a pivot point, such as from catching a skid on an object.
The following videos describe this principle in detail.
Below is a video that includes two examples of a dynamic rollover. The first is when performing sideways flight. The helicopter skids touches the ground the initiates the rollover. The second is the result of poor control on landing.
the following example shows a helicopter landing on a platform. However, the pilot is not successful and one skid falls off the side. At that point, the helicopter is still ok. However, they try to lift off with disastrous results as one skid is caught on under the platform. If they should have shutdown once the helicopter was sitting preciously on/off the landing platform.