Nit'e Gavriel: Shiduchim Us'naim has a whole chapter devoted to the breaking of the receptacle. Selected excerpts:
1. We break a receptacle immediately after reading the t'naim (deal)[1]…
[1] Thus in Minhage Vermayza (=Customs of Worms) 227. See also the book Marg'nisa D've Rabanan… that they broke plates as was customary and he told us "Know why we break plates at the time of the betrothal: It says in Koheles Raba 3:13 that before creating this world God would build worlds and destroy them in the sod of breaking receptacles, so at a betrothal, which is a building forever, we must have, prior to it, the breaking of receptacles, and understand".…
2. The custom is to break a… receptacle of earthenware[3] because of the mourning for Jerusalem.…
[3] … P'ri M'gadim, Mishb'tzos Zahav 560:4, from the Elya Raba :7, so the joy not be complete; it's cited in Mishna B'rura :9.
[4] … Shefa Chayim Nisuin writes a reason we break an earthen plate at the t'naim: Since it's a day of t'shuva and forgiveness, we hint to this with breaking earthenware, "mashul k'cheres hanishbar". And as a way of interpreting the practice of breaking receptacles at the time of the t'naim and also at the chupa (marriage), it seems it's like what they say (B'reshis Raba 73:5) on the verse (B'reshis 30:23) "God gathered my shame", "so long as a woman has no child, she has no one to pin her fault on; once she has one, she pins it on him"… so we break a receptacle, to show that from this match will come many generations of offspring, so that, even if she breaks a receptacle, no harm will befall her, as she can pin it on her child.
3. Some say the breaking of the receptacle is a sign that the match is made….
However:
[16] …the main reason we have the custom of breaking a receptacle is to raise a memorial to the destruction at a time of joy…