an anal fixation generally comes with a cleanliness/filth complex.
on the low end, it can be kids for whom getting shamed by their mother and father for playing outside and getting messy was a regular occurrence. or it can be from getting into the habit of wiping your ass after shitting, not wanting to be shamed for not wiping properly.
on the high end you have molestation and the like
and in the middling area you have stuff like latching onto the asshole as the most 'taboo' sexual organ, particularly guys, and shame of being caught playing with your butthole. you have feelings of being unloved translating into feelings that there's something wrong with you, and being dirty/impure is a common conclusion to reach. there's often a humiliation angle. no matter which way you cut it, licking someone's ass is symbolic of eating their shit, which is considered one of the most humiliating acts you can force someone to do.
it doesn't necessarily stem from a trauma. it can just as easily be the result from repetition which leads to fixation.
shedmon is a particularly entertaining case study in that regard since his anal fixation has an intersection with his mother complex that can be seen from mars. don't know much about blarg's personal life though.
but the tl;dr for nearly any kink is that shame is easily transformed into sexual excitement