I've come across several such issues, but in the steps to reproduce below, I'll be listing the one that can be repeated most consistently. This issue manifests itself in the form of diving to prone (seemingly at random) while carrying out other actions. This includes switching from rifle to sidearm, vaulting (under certain circumstances), simply moving across the battlefield, among others.
- Legacy ID
Start the Editor.
Insert a Rifleman as Player (BLUFOR/Blue/Men/Rifleman).
Click on the Preview button.
go into a low crouch position (the issue does not appear to be present whilst in standard or high crouch)
while jogging forward, hit the vault key (default V)
observe that the player will dive to prone (possibly through/into the object to be vaulted) before standing upright to perform the vault (this can be repeated anywhere, you just won't observe diving into the vaulting object if there's no object to be vaulted over, but moving to prone before standing remains consistent)
See also #446 and #6331.
Another action always triggering the prone issue is lowering/raising your weapon ("Raise weapon" and "Toggle raise weapon" in Options > Controls > Weapons).
Pressing one of the two keys while moving in low stand or high stand, causes the player to go prone and stand up automatically. This doesn't happen in middle stand and middle crouch.
On a side note, pressing either "Raise weapon" or "Toggle raise weapon" in low crouch/high crouch (whether moving or staying still) makes the character go up two stances and remain stuck (as reported in #1148).