That is what is called starke and schwache Flexion.
In case an article determines genus, case and numerus, adjectives have to use strong flexion, if not, weak flexion.
weak flexion uses only the suffixes -e and -en for singular and plural, strong flexion uses all five possible adjective suffixes -e, -en, -em, -es and -er
In the specific case with "es" in combination with a form of "sein", the verb form doesn't come with the singular only, but rather correlates with the substantive form "es" relates to.
Es war meine Mutter
Es waren meine Eltern
The same can, by the way, happen with "das"
Das war meine Mutter
Das waren meine Eltern
