As far as I’m aware this is a unique feature in EBII that attempts to emulate the run up that is required to throw a javelin with any impactful force. As far as I know its not something that can be disabled, so you’re either going to have to get used to it or change your use of certain units and/or your tactics.
I quite like this feature, especially because it does throw my army into disarray a little, forcing me to think on the fly. The javelin or pilla are a very useful part of any Roman army so its not something I personally would want to give up because my formations get all messed up hehe. War and especially battles of this era, are particularly messy. I’m not a member of the EB II team, but I can say with complete confidence that EBII tries to be as historically accurate and realistic as is possible within the MTWII engine. With this in mind, this feature and indeed many others try to reflect this.
I know its not the answer you were hoping for, but I hope I’ve helped in some small way
Cheers,
isa0005