The researchers suspect they've reached a region of the solar-interstellar boundary that nobody had predicted. In this area, the magnetic field lines of the Sun link up with those of the interstellar field. Scientists are calling this linkage a "highway" for particles to travel along. It lets solar wind particles escape more readily, causing the drop in their intensity. And it opens the door for low-energy cosmic rays to slip in to our Solar System, which is why Voyager 1 is seeing so many of them.
According the researchers at the press conference that announced these results, most steady-state models of the Solar System failed to predict anything like this. A few models did have a feature like this, but it was only a transient one that appeared at certain times of the solar cycle.
