every case is a seperate and individual issue.
maybe whatever is causing the problem is close enough to the freq that the HD system is sensitive to, that it only affects the HD system.
the customers on the tower MUST maintain the interference created by their equipment by federal law, one of them could have a problem and not be aware of it because no one has complained yet, and it's not affecting other carriers on the tower or their own equipment.
I'm not saying that something on the tower is totally screwed up, but it may be off just enough to cause a problem with the HD alarm.
I don't care how much "shielding" you try a design into any system, if it's in an area that has high RF interference(like being right next to RF equipment that's generating excessive interference) it's going to have problems.