Unions are typically democratic organizations
Modern democracy relies on secret ballots.Unions want "card check" votes that let the leadership see whom they need to intimidate.
Is a union not just a collection of employees who have entered into a contract together which requires that they do certain things as part of collective bargaining actions.
I don't see why that would inherently require a majority of the employees, or even a majority of the employees in a certain occupation.
Is the 50% requirement due to some enshrinement of unions in law?
Why are unions treated differently than other contracts?
I'm assuming this is because of some historical accident that came about with a struggle between people who wanted to prevent all such contracts and people who wanted unions to have special privileges over other sorts of contracts?
Some union supporters worry that allowing minority unions would weaken established unions. Union opponents have focused on passing "right to work" laws, which require majority unions to represent non-members for free. The status quo probably makes unions stronger in pro-union states and weaker in anti-union states.
Another big thing anti-union propaganda pushes is that it's a vocal minority making a lot of noise. Usually it's the opposite.