D.T.
get her some big girl undies to wear over them or keep her in pants/shorts all the time. This will give you the extra time to redirect her. Little Lady you have a choice, leave that diaper alone or go sit on the potty.
She may be too young to communicate her need so it's up to you to catch her before she does the deed and get her on the potty.
On the other hand she may have some allergic/discomfort reaction to the "cloth" of the diaper once it is wet and needs to get out of it right a way.
My Aunts and Mom say that when us girls (there are 5 of us and only girls) did the same thing it was time to give up diapers all together. Between the 5 of us we have 3 girls and 8 boys. The boys never did this that early but the girls did.