It seems like the game assumes you're always holding an item that can be aimed down the sight, causing you to go into walk mode when you press the optics button(rmb). This doesn't visually change anything, so when this accidentally happens to a player it feels like he is stuck in walk mode.

The same happens when you aim down the sight of a weapon and switch to a different item, it will keep you in "optics mode" making you only able to walk.