It happens because of the custom reload animation for the weapons - and if you are moving when the last piece of ammunition is loaded into the gun, the animation does not finish and you are stuck in a sort of limbo for a few seconds in which nothing can be done.
This is a real nuisance when using these weapons, and it makes them just about unusable.
Is there some sort of fix for this? As I have been unable to find anything that corrects this problem.