I dont know the exact laws but it will be due to the union being paid by a member to act as her/his agent in discussions and disputes with their employer.
The employment tribunl is a lower court specialising in employment law which is why rulings get deferred if referred upwards on points of law or error over applying law to specific facts.
There will be instances where union has agreed X or Y which if fine for 99 of the workers but screws 1. The 100 union members vote and pass the motion. Effectvely the 1 has a new contract T&C which the union signes on her/his behalf.
So it makes sense to streamline the single employee dispute into the ET to determine if the union has the right to bind the employee into the contract with the employer ( employees T&C changed plus job maybe at risk)
Eg an employer recognises the union, the union acts as a block voice for collective bargining to set pay and condition ie a pay rise of £10 /hr and soft blue sofas in the breakroom both conditions are enforcable by the courts.
So the union decision had effectively given the womens employer a blank cheque to disadvantage their gc members without putting the specific event (the film) through any type of democratic process.