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.