I have all the official downloads for Skyrim--Dragonborn, Dawnguard, Hearthfire. Have, in the last 6 months, also dl'ed Falskaar, Wyrmstooth and recently Summerset Isles. Recently the DL order changed by itself

and Wyrmstooth popped up to the top, above the "official" DLCs.

Should I worry that this could cause a problem?

Try to sort your load order again.