You have written that our employees applied mass representation to settle their full and final suddenly.
There are two issues involved in your post. One is that why they felt need to give mass representation on full and final settlement. The question of FnF settlement arises provided they have given resignations earlier. Therefore, please clarify whether employees have given mass resignation. It appears that you have used the phrase mass representation for FnF in place of mass resignation.
The collective action of the workforce implies the collective discontent. You need to find out why they are frustrated so much. The employees are well aware that what individual action cannot achieve that their collective action can achieve hence their collectivism.
You have not mentioned in what capacity you work in the company. If you are from HR then talk to the top boss. With his permission, talk to the employees individually and understand their concerns. Before talking to the employees, talk to the top boss and find out whether he is so self-assured that mass exit of the employees will not impact the operations.
Second one is procedure of the FnF. Even though you have received the resignations collectively, you need to treat these individually. You need to chalk out a plan for their exit. To whom each individual should hand over the charge of the duties, when he/she will hand over, what company property each employee possess, in addition to the monthly salary, what dues are payable and when these will be paid. Discuss with your top boss and explain the procedure to the employees.
From the management science point of view, mass resignations symbolise trust deficit and that too on a wider scale. Your top boss has failed to create an environment of trust. The cauldron of discontent was simmering for long and now the lid has blown off! Legal issues involved in the FnF apart, these are the issues of the organisation's culture and top boss cannot give cold shoulder to it any longer.