I suspected it was an attempt to frame Rideaux in particular, but things didn't line up nearly as neatly as they could have.

[It also didn't stop him from trying to convince everyone it was Rideaux, because Iago is exceptionally petty and did not actually care about voting out the correct culprit.]

A proper attempt to frame someone would be interesting to see, but it would require quite a bit of knowledge about the other person. Ulquiorra certainly wasn't suited to that.