Fresh painted white baseboards turning yellow
Could be a few things. Stains bleeding through the paint from the pine. Chances are, it wasnt sealed properly and you are getting some random bleeding. Could be the paint, Behr is notoriously crap. Depending on the product, there could be some linseed oil in it (even though its latex) If it wasnt stirred during use, it could be some concentrated spots of the oil yellowing. Read the can ingredients on the back.
It could say something like this
Linseed Alkyd Resin <---thats what you're looking for ( or something saying "Linseed" )