Why do the infielders move up sometimes?

Answer 1

It all depends on the situation. If their is a runner on third and it's a close game you would bring the infielders in, so any ground ball hit, would not allow the runner from third to advance home. Also if bases are loaded with 1 out, you would typically only see the corner infielders move up to get the out at home, while the middle infielders play half-way so they can turn the double play.