As per @kasperd's comment on another question.
Looking at the graph below, it appears that Voyager 2 started at just over escape velocity of the solar system. Just before the Jupiter flyby, it was travelling much slower than escape velocity. Presumably, travelling to Jupiter the only thing that would have changed the probe's velocity is the Sun's gravity. As the probe went further away from the Sun, you would expect it to slow down, but why would it drop below escape velocity if it started above it?