Agree. Cichlids are notorious re-arrangers of the substrate, some species much more than others, but most all will do some of this. And sand being easily moved, it gets shifted whereas gravel would be hardly moved from its location but simply "chewed" and spit out. They are looking for food bits in the substrate grains.
Pleco, depending upon species, are active re-arrangers of substrate, or more precisely, they like to dig pits; many do this when spawning, and male pleco will often do this even if no females are present [they are optimistic one will come along and want to be ready]
. I had to place flat rocks in certain spots in my 115g to prevent one pleco from digging down nearly to the tank bottom, some 5 inches of gravel. Here again, sand is easier to move about so it would be more noticed that gravel normally.