Why is lightroom being the default when I open photos in preview on my mac? It has never done it before and I want it to go back.
Don't understand the question. Can you rephrase or provide a screen capture of what you're seeing (and what you expect)?
Authors screen capture
That would be
Lightroom (well maybe, not a Lr user) not Lightroom Classic, but at any rate, reply below still is appropriate.
Well what you're showing appears to be Apple Photo's. If you're double clicking on a raw and expecting LR to open instead, what GoldingD has provided is the fix and reason why.