an alternate hypothesis is that all thinking is unconscious with consciousness being the sum of many unconscious processes. So, while it's true as you say "that some kind of subconscious "brainstorming" taking place", that would just true all the time, and you only notice it when your conscious thoughts are of something else.

(n.b. historically speaking in the literature, "subconscious" was the word used to describe Jung's "woo-woo" ideas about a collective subconscious shared across populations, and unconscious was the word used for ideas about a single brain a la Freud which is what we are talking about here)