I suspect it's more a matter of where the cham tends to be each day when the urge tends to strike. Chams are creatures of habit...they often drink or sleep in the same spots for example. Sometimes they choose a sleeping spot because it is more hidden from view or it's convenient to their heat source when it clicks on the following morning. Like little chams in the wild selecting certain familiar bushes that will get the first sun in the mornings. Obviously, our chams bask in the same spot every day. That first basking session in the morning warms their innards up and that gets things moving. It makes sense that the same spot ends up getting targeted as a dump zone each day.
A stimulating or stressful event can create the urge to drop excess ballast before making a move somewhere else. Birds do that too. When a bird is busy deciding if it should leave a perch, it will often open the floodgates just before launch. Chams and birds are not that different.
A little stress can be a good thing that triggers a poop. And there's good stress as well as bad. Good stress caused by selecting and shooting at a newly filled feeder dish, after a misting & drinking session, being taken to a free roam area or outdoors, etc. I know when I'd carry my chams to favorite basking spots around the house just the stress of being handled tended to trigger a poop once they arrived there. People have daily schedules and that affects their cham care routine. Like when the human shows up to mess around with the cham's territory each day. That in turn creates schedules for our chams.
If I hoped to contain more of the mess, if I put a container anywhere, I'd probably put one below the cham's morning basking area.
Bad stress can obviously trigger a poop too, but it may be harder to predict. It could include being handled for treatment, seeing another cham, a household pet, something outside a window, etc.