S.H.
Is it poop or skid marks?
If skid marks, it can be:
1) she is not wiping well
2) "Encopresis." Which is an involuntary leaking of poop. The person CANNOT control this. You need to see a Doctor. It is sometimes, caused by Constipation.
Also, stress. Can sometimes cause accidents.
And if it is, do not punish/scold for it. Because, it is emotion based.