Why are my some of my LED lights flickering?
Simply put, you have the wrong type of switch for the bulbs you have installed. Most (not in every scenerio) LED bulbs require you to install an ELV type dimmer. What you have installed is a standard incandescent dimmer. 

There are three main types of dimming switches. Incandescent, CFL (for incandescents, fluorescents, and SOME LED bulbs), and ELV (for most popular LED bulbs). 

If you look on the box that the bulb came in, somewhere in small print it should tell you that A) It's dimmable and B) what type dimming switch it requires.

I could go into detail on exactly why sometimes they flicker and other times they don't but I don't think you'd be that interested in learning the engineering aspects of these switches.

Other causes of flickering:

1) mixed bulb types (ie. incandescent and LED on the same switch)

2) mixed bulb vendors

3) non-dimmable LED bulbs

4) defective LED driver

5) defective dimming switch