ajanders
Explorer
I'm thinking that he saw something in the stars that frightened him? or confused him perhaps and so he's secluded himself to work it out properly. Why he has to hide from everyone to do so is still a mystery to me.
Y'know, I'm gonna refer you to Schrodinger's Piratecat. Don't work the answer out. Every time the PC's ask, the cleric has a panic attack and gibbers. Just give the PC's another plot or two to play with. At some point one of your players will come up with a really good explanation that links in with everything else in the campaign they've considered important.
Use that explanation. They'll think you were using brilliant foreshadowing technique and feel very clever for having worked out the mystery. You don't have to tell them the truth.