Involuntary Musical Imagery (INMI) or "earworms" describes the experience whereby a tune comes into the mind and repeats without conscious control. The present article uses an inductive, generative, grounded theory-based qualitative analysis to classify reports of everyday INMI circumstances, and creates graphical models that determine their relative frequency within two population samples; listeners to the BBC radio station 6 Music and an online survey. Within the two models, four abstract categories were defined that described the characteristics of the circumstances surrounding the onset of INMI episodes; Music exposure, Memory triggers, Affective states, and Low attention states respectively. We also note the variety of musical media by which exposure to a tune results in an INMI episode and discuss the impact of musical engagement on INMI experiences. The findings of the present study are considered within a framework of involuntary retrieval theory from both the autobiographical and semantic memory literatures. In addition, the results highlight the potential facilitative effects of varying affective and attentional states on INMI episodes.