Uhhhhh, wouldn't solar radiation be trying to speed it up and wouldn't gravity be trying to slow it down?
Maybe the solar radiation (outward force) is less than expected at that distance and therefore gravity (inward force) is having a greater effect?
Actually, the answer is that these probes are far enough away from our Sun that they have entered an area of space that is less dense. Their speed has not changed, yet the instruments relay back to us a difference, because the instruments are not designed to adjust to a difference in the ‘density’ of ‘space’. The noted ‘change’ then is interpreted as a slowing of ‘speed’.