AFAICT, the implementations have usually been closed-source (with the exception of A+ (http://www.aplusdev.org/), which is GPL'd; Arthur wrote it before K), and most research on APL implementation is locked behind the ACM paywall.
I suspect part of the style comes from APLers typically being mathematicians as well as programmers, and drawing more from mathematics than typical "software engineering best practices" style.
Then, forgive me for harping on this, how could this discussion have already happened numerous times on HN? And by "this discussion" I mean the C style used to implement APL-derived languages, not the style one uses to write programs in APL-derived languages.
I suspect part of the style comes from APLers typically being mathematicians as well as programmers, and drawing more from mathematics than typical "software engineering best practices" style.