This simplified macro:
<assign|bogus|<macro|items|<get-label|<arg|items>><if|false|<bogus|x>|<arg|items>>>>
gives these errors on the console when entered into the editor
.../TeXmacs/progs/kernel/library/list.scm:100:12: In _expression_ (cdr (reverse l)):
.../TeXmacs/progs/kernel/library/list.scm:100:12: Stack overflow
the error vanishes when the first branch of the if block is removed.
I would guess it is over-optimistic reduction of the "if" statement when encountering recursion.
Sam