http://www.tvtechnology.com/article/accidental-discovery-through-testing/272723
TLDR;
ATSC tuners that are not powered up do not have back bias voltage applied to junctions of diodes or the RF amplifier transistor, so incoming VHF signals drive the unbiased junction that is now a highly non-linear load into conduction back feeding third order intermodulation that can interfere in the reception of other tuners within the coaxial RF network.
This interference is channel specific and only applies when the incoming VHF channels third order intermodulation byproduct is in the same spectrum as a desired UHF channel.
Please note that this will become more of an issue when/if the FCC repacks the television channels into a smaller spectrum configuration.