It's generally to prevent free-riding. Let's say the union negotiates benefit X, and the company then provides benefit X to all employees including non-union employees. An example could be safety features that the union is pushing for. The non-union employees have benefited from the actions of the union without having to pay the cost.
To balance this, involuntary unionization usually requires a majority vote from the employees to unionize.
To balance this, involuntary unionization usually requires a majority vote from the employees to unionize.