  We present systematic analysis of spatio-temporal evolution of sources of
hard X-ray (HXR) pulsations in solar flares. We concentrate on disk flares
whose impulsive phase are accompanied by a series of more than three peaks
(pulsations) of HXR emission detected in the RHESSI 50-100 keV channel with
4-second cadence. 29 such flares observed from February 2002 to June 2015 with
time differences between successive peaks of 8-270 s are studied. The main
observational result is that sources of HXR pulsations in all flares are not
stationary, they demonstrate apparent displacements from pulsation to
pulsation. The flares can be subdivided into two groups depending on character
of dynamics of HXR sources. The group-1 consists of 16 flares (55%) with
systematic dynamics of HXR sources from pulsation to pulsation with respect to
a magnetic polarity inversion line (MPIL), which has simple extended trace on
the photosphere. The group-2 consists of 13 flares (45%) with more chaotic
displacements of HXR sources with respect to an MPIL having more complicated
structure. Based on the observations we conclude that the mechanism of flare
HXR pulsations is related to successive triggering of energy release in
different magnetic loops. Group-1 flare regions consist of loops stacked into
magnetic arcades extended along MPILs. Group-2 flare regions have more
complicated magnetic structures and loops are arranged more chaotically. We
also found that at least 14 (88%) group-1 flares and 11 (85%) group-2 flares
are accompanied by coronal mass ejections, i.e. the majority of flares studied
are eruptive events. This gives an indication that eruptive processes play
important role in generation of HXR pulsations. We suggest that an erupting
flux rope can act as a trigger of energy release. Its successive interaction
with different loops can lead to apparent motion of HXR sources and to a series
of HXR pulsations.
