When you stay still while prone, or while stood up with your weapon lowered, your character will perform bunch of random idle animations such as checking his shoe or shifting position slightly.
If you lower your weapon while crouched, you'll probably see your character start inspecting his weapon almost straight away. If you nudge your mouse the animation will cancel and 9 times out of 10 the same animation will start again, this can happen multiple times in a row and looks silly.
I think these "variety" animations play too frequently, and too short a time after being idle. They should only play after the player has been idle in one spot without moving for a set amount of time, say after 15 seconds.