Skulls are almost ubiquitous in pop culture. They occupy my conscious as well. Maybe it was an early obsession with Skeletor on He-Man (although She-Ra's villain, Hordak is a million times cooler) or seeing sugar skulls around the day of the dead. Add an unhealthy dose of metal and you end up with skulls everywhere.
This was just a sketch I didn't really think about but upon examining it I think it has elements of all these. Maybe I'm reading too much into it, but whatever the reason, my obsession with skulls and their prominence in pop culture are stronger than ever!