Fast surface states are found near the seminconductor/insulator
interface.  These states can rapidly acquire charge as the potential
well fills.  However, when the well empties, these states do not
easily release this charge.  

Consider the following (refer to the figure):

When a voltage is applied to electrode 2 the electrons will
move into that vacant well.  The movement of electrons creates
a drift component caused by the lateral electric field (this
is a reflection of variations in the surface potential).  There
is also a diffusion component caused by any gradient in the
concentration of carriers.  This leads to two problems

1.  if the voltage on gate one is reduce too slowly then
the potential well under the gate retains charge

2.  if reduced too quickly, majority carriers will return
from the bulk to the surface and recombine causing a serious loss
of charge.