Why do stocks go down around 1pm?

In my two years now of following the stock market literally every single day I've noticed a pattern of around 1pm stocks seem to go down a little.

What causes this?

I'm not sure it happens every day, but I notice it quite a bit at around 1pm or so.

For example on a rally day, stocks will rally and then around 1pm seem to change direction, only to resume rally later in the day.

Just wondering. Maybe there's no rhyme or reason to it and it's just me.