As mentioned in the comments, this change in behavior can have a medical reason and you should get your cat checked by a vet, just in case. A general checkup is always a good idea for a young mother.
Then you should offer more litter boxes. Yes, she has the whole world to poop, but this is her home. And there are several other cats who poop in her home (even though you clean the litter box, cats have such fine noses that they can still smell the remains).
My theory is that the mother is starting to wean off her kittens. This is a natural process where her motherly hormones get slowly dialed back and her normal cat behavior returns. Those hormones made her love and care for her kittens, but as they get less and less, she will love her kittens less. She will even start growling at and being mean to her kittens in an attempt to chase them away and make them find their own territory. Again, this is 100% normal cat behavior. You should make plans to rehome the kittens.
One possible symptom is being extra dominant by not burying her feces or pooping outside the litter box. That way she marks her territory with her smell, loud (or stinky) and clearly.
Another possibility is arthritis, but that usually happens to older cats. Arthritis or any other painful condition of the joints makes it difficult and painful for cats to climb into tall litter boxes, so they don't climb in but relieve themselves right next to the box.
Yet another possibility is that she has or had negative (painful) experiences with the litter box. That could be an UTI, urinary stones, bad diarrhea or anything else that causes her pain while relieving herself. She may connect the pain to the object (litter box) rather than the action (relieving herself) and therefore avoid the object.
The last two are just some reasons why it's a good idea to get a cat checked by a vet when it suddenly changes its behavior.