Something I do not get

When a crusade starts and when the target provinces is taken-over by anothe catholic faction before it is reached by the crusade it seems that sometimes it is disbanded (with apparently the same loss of influence as for a failed crusade) sometimes it keeps on going and you get a message asking you to hand over the province peacefully when the crusade arrives (if you say yes they ransom your entire army... )

Is there a reason for this or is it just random ?

Likewise, you are sometimes prohibited to start a crusade for which another crusade is already on its way but it is not uncommon to see an AI faction starting a crusade for the same province as you after you've launched you own crusade ... Same question ... Is there some kind of logical explanation that I would have failed to spot ?