Happens adverbs are provided in this article. The words listed here are commonly found along with the verb happens in sentences. This reference page helps answer the question what are some adverbs that describe or modify the verb HAPPENS.
accidentally, actually, automatically, certainly, chiefly, commonly, constantly, continually, early
easily, especially, eventually, exactly, fairly, finally, fortunately, frequently, generally
gradually, immediately, inevitably, infrequently, invariably, mainly, merely, mostly, naturally
necessarily, normally, occasionally, ordinarily, particularly, precisely, primarily, probably, quickly
rarely, really, regularly, relatively, repeatedly, simply, simultaneously, slowly, spontaneously
suddenly, typically, ultimately, unexpectedly, unfortunately, unfrequently, usually
Hope this word list had the happens adverb you were looking for. In addition to adverbs used with happens, other example verb adverb pairings can be found on this site.
You can help by adding more adverbs in the comments. Thanks.