Flapping /16 prefix, why?

Sorry for littering NANOG with this email, but I'm hoping for a little
education on this matter from some of the larger National Service

This morning we had a customer call in concerning a network hole.
Upon investigation, I found the following..

BGP routing table entry for, version 541808
Paths: (1 available, best #1, advertised over IBGP, EBGP)
  1793 1239 1790 3505 3505 from (
      Origin IGP, metric 0, valid, external, best
      Community: 1
      Dampinfo: penalty 656, flapped 18 times in 02:19:29

Two immediate things came to mind here.

1. This is a /16 that is flapping.
2. The end user is using AS path prepending for some reason.

With these two facts in mind, the following questions arise.

Anchoring or tying down of a route with a static route to null seems
to be a very basic idea (far more basic that AS path prepending), so
why isn't this route tied down?

I only see one route to this prefix, why are they prepending? (I admit
that only the user of this prefix can answer this question, and I also
admit that with 53 flaps on the books so far, the other provider may
be dampening the route for us *thank you*)

Finally, does Sprint not apply *any* dampening to customer routes?

My cat is long dead (10 years at least), so it's safe from my curiosity.

Chris A. Icide
Nap.Net, L.L.C.