  Over the past few years, several occasions of large, continuous rotations of
the electric vector position angle (EVPA) of linearly polarized optical
emission from blazars have been reported. These events are often coincident
with high energy gamma-ray flares and they have attracted considerable
attention, as they could allow one to probe the magnetic field structure in the
gamma-ray emitting region of the jet. The flat-spectrum radio quasar 3C279 is
one of the most prominent examples showing this behaviour. Our goal is to study
the observed EVPA rotations and to distinguish between a stochastic and a
deterministic origin of the polarization variability. We have combined multiple
data sets of R-band photometry and optical polarimetry measurements of 3C279,
yielding exceptionally well-sampled flux density and polarization curves that
cover a period of 2008-2012. Several large EVPA rotations are identified in the
data. We introduce a quantitative measure for the EVPA curve smoothness, which
is then used to test a set of simple random walk polarization variability
models against the data. 3C279 shows different polarization variation
characteristics during an optical low-flux state and a flaring state. The
polarization variation during the flaring state, especially the smooth approx.
360 degrees rotation of the EVPA in mid-2011, is not consistent with the tested
stochastic processes. We conclude that during the two different optical flux
states, two different processes govern the polarization variation, possibly a
stochastic process during the low-brightness state and a deterministic process
during the flaring activity.
