Nitei Gavriel - Shiduchim V'Tanaim - page 360 - 32:4 actually brings a few different Minhagim.
1 - The mothers break the plate
2 - The fathers break the plate
3 - The Chassan and the two fathers break one plate and the Kallah and the two mothers break a second plate.
He does not give any reason as to why those that do it in any particular fashion, do it just so. I do agree that most people that I am aware of the two mothers are the ones doing it.
On page 359 column 3 he mentions in the name of the Shefa Chaim that the reason dishes are broken by the Tenaim and the wedding based on Braishis 30:23 where Rachel said "Osaf Elokim Es Cherposi" the Medrash in Braishis Rabbah 73:5 explains that so long a woman does not have a child there is no one to blame for anything that broke. Once a lady has a child she can always say the kid did it. Therefore we break them to show that from this Shidduch will come many children and she will have whom to blame for the broken things in the home.
My own thoughts: Perhaps based on this reason traditionally the mothers are the ones that break it, since it is indicating something specific for the woman.