McHenry, Intermountain, Accurail, and Kadee. The reason is not obvious. It becomes apparent with cars that have metal couplers. The chassis of the locomotive may be common to one rail or another. The coupler trip pin can contact a rail when crossing a rail switch or crossovers.
Brass cabooses with lighting or passenger cars are obvious sources of trouble.
In the case of lighted cars, it is easy to say ensure all your hot chassis rolling stock and locomotives are attached to the same rail. I don't know that this is an attainable goal. Just because a locomotive has a metal chassis or body, doesn't mean that it is common to either rail or either motor lead . Installing any decoder, you know this already. I'll start checking for this as I do decoder installations.