Strategy on castle (short or long) As we know, we usually doing castle to move king to (assumed) safe place.

But when we play short castle or long castle since my chess engine (Houdini) seems to prefer short castle when I play against a strong player and long castle when I play against a weak player.
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.