Why "ce sont devenues"?

Carl,

‘ Ce ‘ is the subject in the expression ‘ ce sont …’ - it is an invariable pronoun here, and there is no plural form of the pronoun ‘ ce ‘ itself. Consequently, there is no expression ‘ ces sont .. ‘.

It is the expressions as  ‘ C’est .. ‘ and ‘ Ce sont .. ‘ that denote singular or plural form of the noun being represented.

 C'est, ce sont = this is, these are (French Demonstrative Pronouns)

‘ Ces ‘ is a plural form demonstrative adjective - it does not exist as a pronoun, and cannot be the subject of the verb. 

Confusion arises because ‘ ce ‘ as a demostrative adjective is a singular masculine form of the group ‘ ce, cet, cette, ces ‘, as noted in the link below. 

Ce/cet/cette and ces = this/that and these/those (French Demonstrative Adjectives)