So why the heck did they rename Occidental to something that differs by one letter from another language that they must have known about?!? Just totally baffling.
As for the name change, I think it has a very general meaning, like amongst-language with the e ending being typical for Occidental nouns and the a ending for Interlingua nouns (maybe). Weirdly enough, the name push was by a guy called Ric Berger who was a Occidentalist who moved to Interlingua and probably played a part in naming both languages. BTW, both languages are very similar in vocabulary roots with Interlingua drawing inspiration from Occidental during its development (if I understood correctly).
First thing that springs to mind: So maybe this Berger guy was trying to force or at least encourage a merger?