Some weapons will make you move slower while equipped to simulate their weight. E.g: The KwK 36 tank cannon will slow you down if you are not in Power Armor.

If you have a bipod equipped and aim, the bipod will greatly stabilize your weapon, just as it would in real life, but only if you’re prone/crouched and/or are “mounted” on a surface. To simulate that, the bipod reduces your movement speed down to 10%. However, the game engine occasionally will lag out & the speed reduction will still apply for a few seconds after you drop out of ADS. You can either wait it out for a few seconds or if you need to get up to speed right-the-hell-now, start sprinting. Activating one of the scripted functions or just bashing whatever’s in front of you will work too.