In most cases it's the result of the original tape having bled through to the blank lead in portion of the tape. Some have offered that the grooves are spaced too close together and you are picking up information from an adjacent groove. I don't buy into this opinion because the information you hear would be one channel only and reversed from the real music. Also, if grooves spaced too closely caused this it would be audible during silent passages in that cut. Every example I've heard in my library is both channels in their correct relationship. This phenomenon is called pre-echo. I wish I knew the definitive answer to this myself.
Music starts early
I have noticed this for some time but never got around to asking the question. Some LP's that I play, I will hear faintly the music start. Then all of a sudden the music does start. Does that make sense? In other words. At the start of an LP. You will hear very quietly the song start. Then the real song starts at the correct level. What is up with that?