English speakers use a lot of butt-on-fire metaphors: we can say someone’s ass is on fire, that he needs to light a fire under his ass, and even the visual of someone flying by the seat of her pants in a chaotic situation conjures an image (for me) of smoking butt. These metaphors all mean different things, but are (appropriately) all descriptions of intense situations (or attempts to intensify a situation, in the case of lighting a fire under someone).
Conceptual metaphor theory (CMT) suggests that we actually understand many concepts and experiences in terms of others. For example, our understanding of time relies on an understanding of space, and the way we think about love is often based the the way we think about a journey. Our language can reflect these conceptual metaphors, as in the deadline is approaching, the best is ahead of us, our relationship is rocky, or referring to an anniversary as a milestone. According to proponents of CMT (George Lakoff is probably best known), we also think of anger as heated fluid under pressure. Angry people might blow their top, explode, or have steam coming from their ears. I don’t know whether we really do conceptualize anger as a heated fluid under pressure, but if we do, it’s interesting to think that the heat isn’t confined to escaping from our head – all orifices seem to be fair game.
P.S. I made the mistake of Googling “ass on fire” while writing this. Bad idea.