Happening adverbs are provided in this article. The words listed here are commonly found along with the verb happening in sentences. This reference page helps answer the question what are some adverbs that describe or modify the verb HAPPENING.
actually, constantly, continually
currently, daily, definitely
finally, lately, locally
probably, quickly, really
simultaneously, truly
Hope this word list had the happening adverb you were looking for. In addition to adverbs used with happening, other example verb adverb pairings can be found on this site.
You can help by adding more adverbs in the comments. Thanks.