I realize this thread is a month old, but I couldn't find a more recent discussion of the issue.
One of the changes in 3.0.8 that we may or may not have announced is that we changed Judgment of Light to do no threat in order to fix some of this seemingly random behavior. We aren't trying to nerf Prot paladin threat overall with this change.
The topic came up again recently when Protadin OTs were accidentally pulling off the MT with huge threat spikes while tanking adds etc.