Ground Contact is defined as this:
Refers to all player contact with the ground directly related to a specific event or manoeuvre, including landing or recovery after being off-balance (e.g., jumping, diving, leaning, or falling).
So if the reason they fell over is related to trying to recover after making the catch, then it would be a turnover. If they fall over as they are going to celebrate with a team mate, no turnover. Obviously there are things in between that, and players would need to use their best judgment in those situations.