Why do my Latent Dirichlet Allocation Topics mix words that never co-occurred?
It could be partially due to the number of topics you selected but the fact that two words rank high for a given topic doesn't necessarily mean that the two words will frequently occur in the same document. Consider a topic that generally corresponds to "medicine". You could have a number of medical documents associated with heart disease and a number of medical documents associated with injuries. They may all be linked by common terms such as "diagnose", "treatment", "patient", etc. But you wouldn't necessarily expect that words like "heart" and "aorta" would appear in documents containing words such as "sprain" and "fracture", even though they are all associated with the general topic of medicine. If one were to choose a larger number of topics, those words might be separated into more specific topics (e.g., "heart disease" and "sports injuries").