Inbalance:
I suspect that what you saw was as the result of static charges built up within the cloud, much like the effect of flying past a thunderstorm and was therefore not due directly by the snow itself.
Weather radar will pick up very low or no returns from snowflakes because they are dry and crystalline, as opposed to large raindrops or wet hail.
Ozexpat seems to have a pretty good idea as to why snow may have an effect and it is interesting to note again that this is not due directly to falling snow but to the fallen snow lying on the ground.
I would hope that any reference to the effect of snow on NDB/ADF in JAA questions would be a little more specific, and make the distinction between falling snow and fallen snow.