Spoiler.

So I'm in the catacombs 2nd level.

I find a dwarf in a jail cell named Shorn. He gave me quite a story and offered to join my party.
But wait, I had to load an earlier save where I hadn't met him yet. When I went into his cell, expecting the same thing to happen, instead he just briefly spouts he needs to run off now and does not offer me an option to recruit him. I don't recall anything differently I did than before.

What gives?

There's a randomness to whether he joins up with you or not. It's a roll of the dice. You'll have to load your game and try again until he does join.
For a cleric his stats utterly suck though, so you might want to pass on anyway.