I do not care much for this question. This is too long for a comment, so here are my reasons.

1. What is the rationale behind such appendices? The single, most accurate answer is that it depends on the paper/authors. For that matter, what is the rationale for appendices in general?
2. What is the complete list of authors of a paper with an appendix? Do your homework here. There is a standard way to cite those papers, which is Authors X, Y, Paper Title, Journal, with an appendix by Z. It works.
3. My impression is that the number of such appendices has been increased drastically in the recent years. Is it true? If yes, what may be reason(s) for that? The first question is factual and can easily be checked using a database. Answering the second question will be purely speculative and I don't see any value in attempting to answer it.