I guess there are so many possibilities how could this happened but I'm thinking you could just limit the participants here or close the application if you reach a certain number of participants.

This could easily be done by bots, or maybe by the managers. But anyway the bounty could easily be managed by the manager so he could do remove a lot of participants here. I guess this is just normal at these times in a bounty campaign so I'm not really surprised what is happening here.