The guardrail that gave way?
The father who opened the window earlier?
The mother who moved the ottoman too close to the window?
The gate that inexplicably stopped being baby-proof that night?
The nanny who ran into the other room to grab his bottle?
The parents who were away at a colleague's baby shower?
The decision to buy an apartment on the 15th floor?
The gusty winds that day?
The decision to go to the party?