The side selected for castling is generally related to the openings chosen by the player. Long castling has the following attributes:
- lands a rook on a center file where short castling does not.
- takes more time since an additional piece must be moved (the queen).
- leaves the pawn on the a-file undefended.
Finally, when the players castle on opposite wings, the game is less likely to be a draw - the opposing pawns can become purely offensive weapons.
Since Houdini is rated probably close to 1800 points higher than I am and because we don't have any specifics, I can't tell you why it's making its castling choice. But it does not move based upon the weakness of its opponent, per se - it is making moves based upon board position.
You'll might see Houdini castling long if it has time to get the queen off the back row. Or perhaps if it can gain a tempo by castling long and hitting an opponent's piece in the center.