got my attention by splashing a tail-load of water on me as I passed underneath the gutter

Apparently the angle of the gutter is wrong for drainage so there is water in there after rain until it all evaporates. Combined with leaf-fall it makes a much-used pond and probably insect-habitat, as tui, blackbirds and mynah not only drink from it or bathe, but frequently help clean it out, dropping the rotting vegetation onto the drive below where it can be picked up and put around plants. Nice work.

These gutters were professionally cleaned about a month ago but there are lots of trees overhead and within a wind's-breath.