It happened on 11 May 1999…nobody knows why and the event was not related to well known drivers of solar weather such as coronal mass ejections or large flares.
According to Durgess Tripathi at the University of Cambridge, UK, and pals, the cause seems to be linked to the appearance a few days earlier of a coronal hole, where the Sun’s corona appeared darker than usual. Exactly how this could have reduced the solar wind to nothing isn’t yet known.
Ref: http://arxiv.org/abs/0807.2697: The Solar Wind Disappearance Event of 11 May 1999: Source Region Evolution