They're trying to find a place that's good to go, I suspect, and not having much luck. Are you properly digging over the places where you want to encourage them to go so that the earth is nice and friable? (Good and easy to dig and cover over - tree root ground can be pretty hard and tough to the paw.)
Big problem with our two outdoor cats (8 months) pooing in the earth beds adjacent to the front door.
Obviously this is not behaviour we want to encourage as it's disgusting and stinks. There are plenty of far more suitable places in the shady bush lined back garden beds.
I've tried everything - citrus peel, cayenne pepper etc. last night I even went to the extreme of completely lining both beds and covering them with layers of slate garden stones. This morning I came out to find poo in the stones?!