They are "neither" random nor illogical.
Same when a Soto teacher approaches a Koan, for example Master Dogen. However, it may be a "logic" all its own, or something beyond discursive or analytical reasoning.
In fact, the only real difference between the so-called "Soto" approach to Koans, and what may be found in the Kwan Um or Japanese Rinzai schools is that, in Soto, the Koans are not necessarily approached as a curriculum that must be passed (same for the Harada-Yasutani lineage, which is a mixture of Soto and Rinzai practices).
http://books.google.com/books?id=-kut6g ... &ct=result
and we do not hold on to, or focus our efforts into, or seek to pierce the Koan during Zazen (apart from the Genjo Koan, which might be thought of a the Koan right before our eyes, and which is our eyes too). In Soto, Koans are approached with a pliant Zazen mind, but off the cushion proper.
Many paths up the mountain. Anyway, what mountain?
Gassho, Jundo