Not being responsible for the merges, I couldn't tell you the exact reasons, but I can surmise the reasoning behind these and I'll offer my guess here.
As it were, the concepts here are quite similar. origin and etymology both cover the origin of a word or phrase, while pejorative and offensive-language both cover negatively-inclined word choice. While one could consider that there is a distinct difference between what is a pejorative specifically versus the whole of offensive language, is that a distinct enough difference that it warrants a separate tag?
There was an earlier discussion in chat we had about tagging articles, which we have three tags for: articles, indefinite-articles, and definite-articles. We all know and acknowledge that indefinite articles and definite articles are different constructs, but, I'll quote myself from chat with some added emphasis,
[...] is there an important enough distinction to warrant separate tags for them?
In the scope of how the site can grow, is there value in keeping the questions about definite articles and indefinite articles separate on the tag level? Or is it more than sufficient to contain both of them within a singular articles tag?
The lack of an actual merge is due to the fact no one who was present at the time of this discussion actually had any ability to vote up the proposed synonyms. Probably should amend that at some point.
My understanding is that the approach to these two merges is the same - users believed that the concepts represented in the two separate tags, while indeed can be identified distinctly, did not warrant a good utility as being separate tags.
If you believe that this should be unmerged, it would be wise to make a strong case as to not simply why the tags represent different things, but why their being separate (and thus fragmenting the results for the otherwise super-tag) is beneficial to the site's utility.