It could be leaching minerals from your media... But also take into consideration if you are tray watering you are putting in water that has 10ppm of minerals. When the water evaporates, those minerals are left behind. Then next time you add 10ppm water, that adds to the minerals that were left behind previously... Over time this number will rise. You can take the plants out of the tray and rinse it out, you may want to top water a little bit to rinse out the pots as well while you are at it. I'm not sure I'd be worried at 75ppm though. Not yet

Maybe I'll run out and check some water in my Sarr. trays... I've not rinsed them in a loong time :X
Hope all is well Phil!
Andrew
---------- Post added at 09:11 AM ---------- Previous post was at 08:25 AM ----------
Hmm... my ro is about 14ppm and one of my Sarr trays is reading 38ppm, and I cannot recall the last time I took all of the pots out of it and rinsed it.

Maybe yours is having dust blow into it adding to the tds? I'm lost. Sorry bud.
Andrew
Edit:
Or maybe the green stuff that randomly shows up in my trays consumes the TDS and that's why it ultimately ends up dying off... once it has consumed all the TDS lol