Why do some LED light bulbs flicker?

When an LED light bulb is receiving just under its activation level of energy, it will flicker. Unlike incandescent bulbs, LEDs require a specific amount of energy to turn on. This is known as the activation level.

Lower-cost LEDs have a higher activation level. This, when combined with the minimum amount of power provided by certain dimmer switches, causes the LED to flicker. 

For a list of recommended LED light bulbs, see Which LED light bulbs does Alarm.com recommend?.