At the moment, I got some info. Such morphing happens in catas, and partially in other areas. I think I 've never seen anything go weird in abyss and hell. but all other areas can play jokes with this. the worst case is still catas. It can happen on ALL difficulty modes and both in SP and MP.
What I did find out recently was that the actual AMOUNT of morphing is somehow seems to be connected to monster seeding size values. Even in Caves, when I was playing around with seed values, one time I remember going to DLVL 10 and seeing there a bizarre combination of doors and wood grates. Some of those grates ended suddenly and doors were close to each other. One particular grate stood just outside of another woodgrate room and had a door in it I didn't save game on that level, but before entereing it, by the stairs (on DLVL 9). When I restored seed values to previous values, then loaded game entered the level, no such things would occur.
I'm also wondering if that might be connected to monster placements in the table. Originally, Diablo had a neat monster table, where those would start with level 1 monsters and progress to the monsters of level 24 (Crypt level 4). No doubt, it's a wild guess, but.. I have to think something.