I think it's a mistake to think and to base reason on the thought, that all things that happen in our mind are reasonable, rational, and part of a neat cause-and-effect global (to the mind) order.
While that may be true, it is also true that microscopic changes also affect the mind, which microscopic changes have no connection to the order of the mind's world, but occur due to environmental changes; such as chemically (and not cognitively) induced firing of neurons, which have a cause, (chemical, not mental) and the effect is the upbringing of an unrelated memory.
Randomness happening in the mind is rampant, and creative types get more of it. The entire process of problem solving hinges upon randomly connecting or trying new connections, of explanation. If they are wrong, they get discarded; if they are right, they get reinforced and become a new pattern recognized by the mind.
So don't expect that all memories are and need to be connected. They can and do pop up, because part of the survival tactic of the mind is to produce seemingly random connections (which need to be relaity-tested, but their force of usefulness is exactly their randomness.)