Why do clean clothes smell when hung on the line?

Are they hung inside or outside?

i suggest that you re wash your clothes add a cup of vinegar to the wash cycle with your soap. Then dry them on the line no bunching. I often find that in the center of the washer is a spot you can pull out and clean. There is a trap that must be cleaned regularly. Especially when you clean wash smells funky.

I second the white vinegar suggestion, but add to the rinse water. I add mine to the fabric softener dispenser for natural softening and deodorizing. If you use fabric softener or scent enhancers the laundry might need a deep clean.... especially towels. Here’s my method: https://brendid.com/how-to-naturally-clean-smelly-towels-the-fool-proof-guide/

If you're washing clothes in a front loading machine, it's most likely building up mold inside the gaskets.

Wash the washer with an appropriate item available at hardware or grocery stores, then always, always wipe out the gasket areas with a dry cloth, prop open the washer door (at least 6"), and fully open the compartment door where you place detergents and fabric softeners.