(Note: this is based on what I found in literature. Sea ice is not my expertise.)
Short answer: We don't know.
It may be related to changes in atmospheric temperature, wind stress, precipitation, ocean temperature, changes in coastal polynas, or other factors. The usual way to explain observed behaviour is through models. However, models are currently unable to represent the observed behavior.
The IPCC AR5 WG1 report, chapter 4 (Vaughan et al., 2013), addresses Antarctic sea ice. Firstly, as your question addresses overall winter sea ice, it is important to note that trends vary as a function of season and location:
The seasonal trends are significant mainly near the ice edge, with the values alternating between positive and negative around Antarctica. Such an alternating pattern is similar to that described previously as the Antarctic Circumpolar Wave (ACW) (White and Peterson, 1996) but the ACW may not be associated with the trends because the trends have been strongly positive in the Ross Sea and negative in the Bellingshausen/Amundsen seas but with almost no trend in the other regions (Comiso et al., 2011). In the winter, negative trends are evident at the tip of the Antarctic Peninsula and the western part of the Weddell Sea, while positive trends are prevalent in the Ross Sea. The patterns in spring are very similar to those of winter, whereas in summer and autumn negative trends are mainly confined to the Bellingshausen/Amundsen seas, while positive trends are dominant in the Ross Sea and the Weddell Sea.
(a) Plots of decadal averages of daily sea ice extent in the Antarctic (1979–1988 in red, 1989–1998 in blue, 1999– 2008 in gold) and a 4-year average daily ice extent from 2009 to 2012 in black. Maps indicate ice concentration trends (1979–2012) in (b) winter, (c) spring, (d) summer and (e) autumn (updated from Comiso, 2010). Source: Vaughan et al. (2013)
A recent article on the topic is by Holland and Kwok (2012). In the abstract, they summarise:
The sea-ice cover around Antarctica has experienced a slight expansion in area over the past decades1, 2. This small overall increase is the sum of much larger opposing trends in different sectors that have been proposed to result from changes in atmospheric temperature or wind stress3, 4, 5, precipitation6, 7, ocean temperature8, and atmosphere or ocean feedbacks9, 10.
The article itself is behind a paywall. Back to the IPCC AR5 WG1 chapter 4 (Vaughan et al., 2013):
An increase in the extent of coastal polynyas in the Ross Sea caused
increased ice production (latent heat effect) that is primarily respon-
sible for the positive trend in ice extent in the Antarctic (Comiso et
al., 2011). Drucker et al. (2011) show that in the Ross Sea, the net
ice export equals the annual ice production in the Ross Sea polynya
(approximately 400 km3 in 1992), and that ice production increased by
20 km3 yr–1 from 1992 to 2008.
So, the best answer we can give so far is: we don't really know.
Vaughan, D.G., J.C. Comiso, I. Allison, J. Carrasco, G. Kaser, R. Kwok, P. Mote, T. Murray, F. Paul, J. Ren, E. Rignot, O. Solomina, K. Steffen and T. Zhang, 2013: Observations: Cryosphere. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Holland, Paul R., and Ron Kwok. "Wind-driven trends in Antarctic sea-ice drift." Nature Geoscience 5.12 (2012): 872-875.