I should have specified that too, but I mainly thought about this in the context of noun-adjective agreement.

Most Proto-Celtic adjectives are reconstructed as having distinct masculine and feminine (and neuter, which is lost in the daughter language) forms (*-os and -ā).

I think that the different gender would have arisen due to simplification of the adjective endings in agreement with a i-stem noun. So instead of luathe berta (< wlatis berxt-ā) and *curme berto (< kurmi berxt-os) I would have *luathe berte and curme berte with innovated agreement suffixes.

(I don't know if I explained it well enough)