Whenever our cats start doing this, my husband checks for worms as well just
to be on the safe side. It's never showed them to have worms, he's just had
a cat who did that once and it was because they had worms which is why he
always checks.  Here in the desert tho (I live in Vegas) I noticed my oldest
cat Wylie does it a lot more.  I think it's more of a dry skin issue.  Both
our cats have a lot of hair on their back ends.  We shaved it once and next
time we're taking them to the groomers so the groomer can be the bad guy.
Same thing with giving our oldest Wylie a bath.  Wylie has a personality
that when she doesn't want to do something, she ain't doing it. Bath
included and we've bathed them since they were babies, she's just always
been like that.  The youngest though will give in with a more reserved "I'm
already wet might as well get it over with." Wylie is more like a bucking
bronco or bull.  So when it comes to shaving their behind, I definately
recommend letting the groomer being the bad guy. This way when you return
your the hero in rescuing them.  Something a vet told me once when they took
my oldest Wylie out of the room to draw blood (and yes ended badly for the
vet) so that when they brought her back I got to be the hero saving her from
the mean old vet. :)

Just a thought :)

- Leigh