Why does my painted wooden post have brush lines?
That you have never seen brush lines before you have been very lucky. 
All of your ideas can affect how the paint goes on, cheap brushes, cheap paint are big ones but, the main reason I see for excessive lines is over working the paint so it won’t flow.
when I do cabinets I don’t even try a brush but spray the paint and have used additives like floetrol a paint additive when I want glass smooth finishes. 
Sanding with a really light sandpaper will take the shine off, if I am putting a second coat on doors to get that perfect finish I will use 600 or finer paper wet to buff the door prior to the second coat if it is needed. I normally do this with clear varnish several times to get a real depth to the work, but not often on paints. 
You could sand the high ridges and reapply but make sure to over work the paint I see people put paint down spread it out then come back in a minute or 2 and run the brush over it again, this is when the brush marks stay or because of what I was taught is over working the paint.