I've given this one a bit of thought, in trying to rationalise what exactly I find gripping about the topic.
I recall my first visual experience was an episode of Samurai Pizza Cats. The scene itself didn't contain much "sinking" -- it mostly cut between two different plot lines with the main characters getting deeper between each cut, but I vividly recall the character's legs sinking even an inch. For some reason, at that very young age, I was captivated by the quicksand scenario. I remember scenes from Captain Planet, Rugrats and even an ABC show called "Fireman Sam" that featured a plastic-model boy getting stuck in "quicksand". Obviously, at that age it was mostly the "terror" of being stuck that caught me, but there was something definitely there. I recall, in school, we played a "Goosebumps" computer game that featured first-person adventure and puzzle-solving. I clearly remember a section that involved being trapped in a slow-sand carpet, where you sank (slowly, of course) and the slow-sand filled up the screen, only to be escaped by grabbing onto the tail of a giant rat. I played a game on the Neo Geo console (a platform game; the name eludes me now) that featured a swamp/sinking section that required the player to carefully jump over, yet I found something interesting about letting the character sink before jumping out (I have a natural aversion to watching characters submerge, something that runs true today). I also remember watching the foreign-film Fantaghiro, in which the titular character abandons her talking horse, who sinks into dry sand and she is consumed with it (and the immediate scene afterwards, which involves a lot of lemons for some reason). Again, nothing sexual; just drama.
As I grew into my teens, I still felt attracted to the idea. It was still a drama thing, but in hindsight I found myself aroused by seeing characters in quicksand. I had never been in anything resembling it myself, of course, other than the soft sand on the beach (and everyone makes quicksand jokes about making footprints). I increasingly found myself fantasising about people -- real and fictional -- being caught in quicksand traps. Tomb Raider was the big thing that came out, and the third installment featured innovative "quicksand" tiles. I only ever played the first level, which happened to contain quicksand, and I never bothered to go past that level. Just controlling Lara and wading through the quaking surface at different depths was exciting enough. Was it the sucking noises? The sluggish movement? The sense of being trapped? Again, it wasn't a physical or sexual arousal (Lara was, even more so back then, a bunch of polygons).
Fast forward to the late teens and young adult stage (and keeping in mind I'm still relatively young). For some reason, I recall the last book in the John Marsden series "Tomorrow When the War Began", in which the main female character describes quicksand on two occasions -- one metaphorically after running around in circles; another more literally as she has to work knee-deep in mud at a labour camp. By now I had accessed the online world and began my first experience with reading quicksand-orientated material -- Kaol's Ol' Sinkin' Hole was my first contact with the online community, and the links to images and videos opened up a new dimension for me; an experience that most of us probably share.
As an adult, my particular tastes in quicksand material have matured alongside the reasons behind my interest. Whereas in my late teens and early 20's I generally avoided material that was overtly sexual (and my stories at the time were more for the Afternoon/Evening crowd on DS), I am indeed more attracted to the sensual side of sinking. I do feel attracted to the physicality of the female form in quicksand -- often scantily clad (but not always); curves accentuated by the enveloping quicksand; the quiver of the surface; the slowly disappearing body. I feel particularly switched on at certain points -- thighs, hips, waist and chest. The moaning and grunting of the struggle has strong parallels with arousal. While I'm still not found of overt arousal (and, incidentally, I have no interest in mud play at all), I definitely have refined my taste (and my own stories and artwork) towards the sexual arousal side of the fetish.
In short, you could say that my interest in quicksand grew up alongside me.