The reason behind the minhag of breaking the plate is as follows:
In old times, the tenaim were very strictly fulfillrd and breaking these conditions was considered a grave transgression, one that cannot be amended. A kli cheress, pottery, cannot be fixed once broken. The breaking of the plate essentially is announcing that whoever dares break the tenaim should be aware that this transgression can never be fixed. That is why it is done at the tenaim. It is not zecher l’churban.