Why do Random Words Pop Up in Your Head
It may happen sometimes that a word unrelated to any thought processes that you are having or had to just pop up suddenly. I don't know how frequent random process is happening in the general population but for me it's rare, when I'm running it seems to happen a bit more frequent.
I find this phenomenon interesting, my explanation is that it has to do with the unconscious, in recent years neuroscientists showed that the unconscious mind is responsible for much, if not all, of our actions . So what happens is, the unconscious makes a quick, shortcut-type decision based on the patterns that it has and then the prefrontal cortex, the conscious, rationalizes the reason why we did that action. We also know that the mind triggers random thoughts to pop up, the whole basis of meditation is to remove this random thoughts that pop up. We also know from neuroscience that neural network get stronger the more you access them, the myelin sheath that surrounds the axons gets stronger and this in turn makes that particular neural network to fire up faster when required.
How important is myelin? Well, in order to illustrate its importance consider what can happen when you start losing it: "impairs the conduction of signals in the affected nerves. In turn, the reduction in conduction ability causes deficiency in sensation, movement, cognition, or other functions depending on which nerves are involved". My guess is that the brain evolved to fire up random neural networks in the form of thoughts and words as a form of "exercise". This randomness is basically how the brain "works out", it triggers a random thought, that thought triggers in exchange other ones in a thought circuit and so on. It can happen sometimes to have a repetitive circuit of thoughts.
Given the neuroplasticity of the brain which means that the brain tends to allocate neurons from less utilized networks to highly used networks - that's why you start forgetting words and skills that you don't use - the effect of not triggering random thoughts can lead to loss of memory. It's clear why evolution would favor a brain that generates random thoughts over one that doesn't. The hominid with a zen-like, thoughtless mind has a higher probability to forget crucial survival knowledge, skills, which plants are edible, which plants are poisonous, hunting and trapping techniques, how to make fire, where are the water sources, memories of unexpected situations and their outcomes. Such an hominid won't stick around for too long if he has to re-learn things through trial and error more frequently than it should. For example, tribal songs are a great way to transmit and keep knowledge in an oral form. Oral tradition lasted quite long, even long after writing first appeared. Songs are designed to be repeated and mimicked and the word repetition is the key here. There is a wise saying: "Repetition is the mother of learning".
So this would be my theory on this subject: the mind triggers randomness and repetition as a way to consolidate memory and learning.