  We study a possible origin of the anomalous suppression of the power spectrum
at large angular scales in the cosmic microwave background within the framework
of explicit string inflationary models where inflation is driven by a closed
string modulus parameterizing the size of the extra dimensions. In this class
of models the apparent power loss at large scales is caused by the background
dynamics which involves a sharp transition from a fast-roll power law phase to
a period of Starobinsky-like slow-roll inflation. An interesting feature of
this class of string inflationary models is that the number of e-foldings of
inflation is inversely proportional to the string coupling to a positive power.
Therefore once the string coupling is tuned to small values in order to trust
string perturbation theory, enough e-foldings of inflation are automatically
obtained without the need of extra tuning. Moreover, in the less tuned cases
the sharp transition responsible for the power loss takes place just before the
last 50-60 e-foldings of inflation. We illustrate these general claims in the
case of Fibre Inflation where we study the strength of this transition in terms
of the attractor dynamics, finding that it induces a pivot from a blue to a
redshifted power spectrum which can explain the apparent large scale power
loss. We compute the effects of this pivot for example cases and demonstrate
how magnitude and duration of this effect depend on model parameters.
