  A series of five unusual slow glitches of the radio pulsar B1822-09 (PSR
J1825-0935) were observed over the 1995-2005 interval. This phenomenon is
understood in a solid quark star model, where the reasonable parameters for
slow glitches are presented in the paper. It is proposed that, because of
increasing shear stress as a pulsar spins down, a slow glitch may occur,
beginning with a collapse of a superficial layer of the quark star. This layer
of material turns equivalently to viscous fluid at first, the viscosity of
which helps deplete the energy released from both the accumulated elastic
energy and the gravitation potential. This performs then a process of slow
glitch. Numerical calculations show that the observed slow glitches could be
reproduced if the effective coefficient of viscosity is ~10^2 cm^{2}/s and the
initial velocity of the superficial layer is order of 10^{-10} cm/s in the
coordinate rotating frame of the star.
