The beginning of midday is when there is a lot of sun in the world.  This represents time of great wisdom and intelligence. The dinim  that attach and attack that state is a very strong and is hard to recognize. Therefore they have a strong ability to trick a person into following it. That is why the beginning part of midday is called Mincha Gdola since the holiness needed to overcome it is very great.
In the late afternoon close to sunset the evil becomes more apparent and easier to battle.
Rav Nason Likutay Halachos, Mincha 5, 7