Double sieges when enlisted?

[okyman]

I have noticed that while im enlisted in an army, everytime we siege something we have to do it twice....  Then i noticed we were fighting allies on the second one everytime.  So i payed more attention and eventually caught on that the lord im fighting for will participate in the capture of the fief, but right after, we have to defend against all the other lords that were apart of the siege.  So first one is offense, second one is always defense.  Why are we fighting our own faction right after we take a settlement everytime?