If your cat is ok with you touching them and especially stroking the base of their tail, then you must have a very close relationship and mutual trust.
For a new owner, this might be a sign of affection, or pain, and not actually realizing that petting her at the back may be stimulating the instinctive mating response.
Since cats keep some of their early habits throughout their adulthood, so does the raised back can be part of that response to their mother’s grooming.