Having been in the importing game, just a couple of views. Importing rooms here tend to have UV sterilisers connected. To kill off unwanted organisms etc.
Therefore moving them to not so clean tanks can expose them to certain pathogens.
Some species are more prone to diseases than others, due to bad breeding practices, or changes from wild-man conditions.
Some have there own personal problems, neons, gupppies, discus....etc. Some are more prone to common diseases than others, ie; clown loaches to white spot.
Chances are the importer got a batch in where a group of fish weren't right. They just came down with something, could of been a bacterial infection, flukes, internal worms etc. Or something as common as dirty water. Few hundred guppies in a bag from Asia to NZ can cause problems. Also changes in ph, where we ordered from in Asia, they gave us the ranges they kept their fish in.
Frenchy