Because of the motion of the Earth relative to the galaxy. Our Solar System is in orbit around the galaxy, and thus traveling at considerable speed (200 km/s or so) around the galactic center. The dark matter halo of the Milky Way, however, does not travel in the same way, so there is a persistent relative velocity between the Earth and dark matter (the WIMP cloud), this is called the "WIMP wind". However, the Earth itself orbits our Sun in a circular motion, which causes it's velocity relative to the Sun and the overall Solar System to change from about 30 km/s in one direction to 30 km/s in the opposite direction, a difference of about 60 km/s, which is significant compared to the 200 km/s galactic motion. Due to the orientation of the Solar System the relative speed of the "WIMP wind" is highest in June and lowest in December. This has nothing to due with Earth's orbital nodes or with the seasons, it merely has to do with the coincidental orientation of Earth's orbital plane relative to the galaxy. Because Earth's orbit is not aligned with the Sun's orbit through the galaxy (it's actually tilted at about 60 deg.) the difference is not the full 60 km/s, it's closer to just 30 km/s (cos(60 deg.) = 1/2), but that's still significant.
There are more details than you'd ever care to learn about here: http://arxiv.org/abs/1209.3339