That doesn't sound right. Annual Leave and Unpaid Leave is not the same thing. Surely you can't tell everyone in the middle of June "we are shutting down for 2 months, you all WILL go on "Unpaid Annual Leave" ? Just because it's in December doesn't mean it's fair..surely.
It's one thing if you get 20 days leave per year and you use it all up before December and THEN they tell you ,you must take 10 days "forced" leave and THEN tell you it will be unpaid because you already used up your leave [that sounds fair enough] , BUT there's no way i can think it's fair to force you to go on unpaid leave if you never had any leave to begin with and/or they are not giving you enough leave to cover the "forced" leave period.
I wouldn't accept that, since it's open to abuse every-time the company is running low on cash , they just put everyone on some sort of unpaid leave...? Isn't that basically like "suspending without pay" ?
You are right. I did not notice the OP saying that they were being forced to take more leave than they were being granted per year. Unless agreed to by the employees that would amount to unilaterally changing their employment contract.