  GRB 130925A was an unusual GRB, consisting of 3 distinct episodes of
high-energy emission spanning $\sim$20 ks, making it a member of the proposed
category of `ultra-long' bursts. It was also unusual in that its late-time
X-ray emission observed by Swift was very soft, and showed a strong
hard-to-soft spectral evolution with time. This evolution, rarely seen in GRB
afterglows, can be well modelled as the dust-scattered echo of the prompt
emission, with stringent limits on the contribution from the normal afterglow
(i.e. external shock) emission. We consider and reject the possibility that GRB
130925A was some form of tidal disruption event, and instead show that if the
circumburst density around GRB 130925A is low, the long duration of the burst
and faint external shock emission are naturally explained. Indeed, we suggest
that the ultra-long GRBs as a class can be explained as those with low
circumburst densities, such that the deceleration time (at which point the
material ejected from the nascent black hole is decelerated by the circumburst
medium) is $\sim$20 ks, as opposed to a few hundred seconds for the normal long
GRBs. The increased deceleration radius means that more of the ejected shells
can interact before reaching the external shock, naturally explaining both the
increased duration of GRB 130925A, the duration of its prompt pulses, and the
fainter-than-normal afterglow.
