Moving a mat with cargo loaded to a location on the map where existing cargo pieces reside (e.g. a very “busy” map) – the cargo on the mat picks up cargo pieces from beneath the mat, creating a stack on the mat, and loading the pieces that were below the mat onto the mat. Move the mat a couple of times and many pieces are unintentionally added as cargo.
It would probably help to supply the module in question and a specific series of reproduction steps. I can’t visualize what’s happening without knowing the relative sizes of the mat, the cargo pieces, and the map location(s)…all you said was that it’s “busy.”