I'm not a fan of the idea even though it seems to be popular but I find it interesting that there is a theory that there is a submissive quality to it.
According to the book "Curious Customs'' via Dear Abby, it starts with the groom helping the bride's hand, a symbolic demonstration of male control. She accepts this gesture and, as a further proof of her submissiveness, performs the second step of the ritual, offering him the first bite of cake, the gustatory equivalent of her body, which he will have the right to "partake of''' later.
In the third step, the master-servant relationship is temporarily upset, as the bride mischievously pushes the cake into her new husband's face. Significantly, this act of revolt is performed in a childish fashion, and the groom is able to endure it without losing face because it ironically demonstrates his superiority: His bride is an imp needing supervision.''
The bride then wipes the goo apologetically from his face. This brings the play back to the beginning as she is once again obedient to his wiser judgment.
I doubt most couples who smash the cake see it as a submissive act but I thought it was an interesting theory.