I have a very old house and it used to have mold in it. Now mold grows when there is water vapour and no air flow. So it's entirely possible the cheap frame backing you use has mold spores and maybe when you fitted them, as there is no air flow - the mold spreads.
If you perhaps used conservation level mountboard, then the mold issue would not happen. What I would do is, unwrap one of the frames you are using, and just leave it and see if mold appears on the backing. I would also frame something as normal and see what happens. I would also go to a DIY store and buy some bleach free mold killer (the non yellowing kind) - and spray the board and see if anything happens. At least that way you will establish the root cause.
In the mean time I would seriously consider conservation mountboard - it's acid free etc. I use it all the time from my picture mounts
supplier (I know you call them picture mats in the US) they have all the spec on them.
as for the old victorian house we live in - I made every mistake in the book, by sealing up ventillation points , using the wrong type of plaster on the walls etc. Once property vented - the mold problem disappeared