I'm still trying to get a good look (i.e., screenshot) of it pre-change, but it appears that Farsight and Ctrl-J are both too slow to get there in time, I'll just have to enter via the usual method and Boots of Speed it over there. For now, I've Emailed a shot of the rock in question to Andyr.
There's obviously something very unusual about the rock--there's a big crack between it and the other rocks, showing that
a) the rock is very thin, probably little more than a shell, and
b) there's a good, strong light source under the rock.
That, coupled with the fact that it changes from something else, just says "Concealed Cave Mouth" to me. The fact that there's an animation associated with it reminds me of the animations of the various fireplaces in the game, the grinding wheel in Aran Linvail's torture center, and the various throbbing orifices in the Unseeing Eye lair. With all of these animations, as soon as you come within sight range of any portion of the object, the entire object pops into view--this might be what causes the impression of the rock suddenly "appearing" in order to cover up what was there previously.
Ctrl-4 shows no "trigger area" as there is with all doorways that are still operational. It's possible that the only answer might come from examining the .ARE file to see what it looks like without the animation.