It's slightly dependent on the player.. and I think also dependent on the number of items placed and when they were placed.
If you are a stickler for trying to organize items all the time it'll really get screwed up. Sometimes things fall.... just leave them and don't pick them up again and next time the place respawns, everything will be back to normal. If you constantly try to pick them up and reorganize stuff it'll just get worse and worse because the game loses track of the original placement order.
You also seem to reach a point where you just have too much stuff and the game can't keep track anymore. Items I've placed early in the game...everything is fine and has stayed where I put it. Later placed objects have trouble staying put. It's like once you reach a number of items placed.. it just has more trouble keeping track or something. Like magazines even. My first two magazine shelves are great. Nothing has ever fallen off those two. The later ones I started placing after the first two filled up are always screwed up.. magazines all over the floor.