The AI crusades never cease to bemuse me.

I am currently as war with the Spanish and have the provinces of Portugal and Valencia.
The Spanish have wiped out the Almohad and own all the rest of the peninsula all the way to Egypt.

The Spanish launched a crusade against the Egyptians from Castile bound for Arabia.
Now as we all know out player Crusades are required to take the shortest routes to the goal province well I wasn't too suprised when on the first turn the AI crusade attempted to march through valencia.
I denied it access and turned it away.
Next turn it moved forward into cordoba so I thought yep that is better go to your target.
Well I was most suprised when next turn it went backwards into castile, picked up more men from the high zeal province, and the following turn moved into Portugal, where by the way there is no port or current shipping lanes of any nation for the Spanish to use.
I have turned away the crusade again for it to retreat to Castile.
I haven't played for a few days but am wondering what next and is this circular crusade to continue.
I was looking for similiar experiences or insights as to how the AI is justifying it's crusade movement?