The anims/fps/fps_spawn.animation_states file defines which animation should be played for each FPS state, like sprinting or shouting.
The state for shouting 'get down', fps/command/down, has a parameter set up to determine which 'get down' animation should play every time you enter this state:
<param name="var1" value="1 * ( rnd() < 0.5 )"/>
rnd() generates a random number between 0 to 1.
rnd() < 0.5 evaluates to 1 (normal get down animation) if rnd() is less than 0.5, and evaluates to 0 (alternate get down animation) otherwise.
The problem here is that rnd() does not work in animation xml files! Whoever set this up mistakingly assumed it did.
Overkill's animation xml parser seemingly forces this invalid value to be 1, thus the alternate animation is never actually seen.
This mod moves the randomization calculation to lua instead to fix this.