You do not mention it explcitly, but there are also appendices to papers by the same authors. Thus, author-credit can certainly not be the only rationale.
Two other reasons for an appendix:
The content of the appendix are 'just technical arguments' and the authors do not want to clutter the main article with them.
The appendix contains some investiagtions tagential to the actual article but suggested by it.
In particular, 1. is I believe quite common in (some) more applied parts of maths, in particular TCS.
And, point 2. can reasonably be done by a person not the author, and even after the article is essentially complete. Thus, one reason for an appendix can be that one receives significant comments on a preprint, yet possibly on a tagential point. Something like, some technical lemma when considered right is of idependent interest or alike.
One thing that is also worth pointing out I believe is that the general idea that if A has an appendix to papers of B and C, then B and C did not let A 'in' as a main author, is certainly a misconception (as a general reason, in particular cases it might be like this). There are papers where the author of the appendix is by a significant margin the most well-established of the authors, and the idea that the main-authors did not want him/her as a co-author seems strange.
Regarding frequency, I do not really know whether the increase of appendices is higher then the general increase of co-authored papers, it might be due to more rapid communication.
As a final remark: an appendix authored by somebody else is not a very recent (mis-)developpment prompted by the fact that people now care or have to care more about author-credit. By contrast, I think it is a good tradition and a consequence of the fact that paper-count is not (or at least was not) considered overly important in maths.