When looking through the combat rules I realized under Being Prone it states:
You can drop prone without using any of your speed.
There doesn't seem to be a reference that this needs to be on your turn or during your move or whatever else.
Notably, there are ways to move off of your turn such as dissonant whispers or the Ready action.
Can you voluntarily drop prone when it is not your turn (when no other effect is making you fall prone)?