The only explanation is that after calling Crusade and joining, one of your allies allied themselves with the Moors. When an ally becomes friends with your enemy, the game automatically places you at peace with the warring faction. If you sign a peace treaty with the target of a crusade, while still on crusade, the entire army disbands the next turn.

That explains why you had to declare war twice, why your crusade disbanded in a single turn, and why you were forced into being at peace with the Moors right after declaring war.

As far as I know, that's the one and only explanation. Pretty rare and frustrating stuff...