UCLA researchers showed that the missing electrons are swept away from the planet by a tide of solar wind particles during periods of heightened solar activity.

The data show that while a small amount of the missing energetic electrons did fall into the atmosphere, the vast majority was pushed away from the planet, stripped away from the radiation belt by the onslaught of solar wind particles during the heightened solar activity that generated the magnetic storm itself.

http://www.nature.com/nphys/journal/v8/n3/full/nphys2185.html
http://www.indiatimes.com/science/solar-storms-behind-vanishing-electrons-in-earths-outer-radiation-belt-11308.html
http://blogs.scientificamerican.com/basic-space/2012/02/17/dropout-electrons-get-pushed-out-of-van-allen-belt/
http://www.universetoday.com/93172/the-van-allen-belts-and-the-great-electron-escape/