Hurricanes, and tropical cyclones in general, are steered by the larger-scale wind currents that surround them. These winds vary based on the strength and location of the various high- and low-pressure systems surrounding the storm. In the case of Hurricane Bill, the west-northwestward motion has been caused by a strong high-pressure area to the north. The clockwise winds around the high result in easterly winds on the south side, where Bill has been tracking for the past few days. The current forecast is for a trough of low pressure to approach from the west. The winds around a low in the Northern Hemisphere blow counterclockwise, and so the winds ahead of the trough will blow from the south. Bill is expected to feel the effects of this trough by moving more to the north, as indicated on the map. This behavior, called recurvature, is common in Atlantic hurricanes, since the high pressure over the Atlantic (the "Bermuda High") is a semipermanent feature during hurricane season and low-pressure troughs occur frequently.
Source(s):
http://www.hurrnet.com/tutorial/myths/probabalistic/steering.html
http://www.tpub.com/content/aerographer/14010/css/14010_216.htm

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