Probably a PPS member has wanted the seat the you booked and so SQ bumped you. I had a similar experience.

A few years ago, I was on an AC reward ticket flying in J on SQ. I had an aisle seat reserved to avoid sitting in the middle seat of the centre three seater on a 777. At check-in, I was bumped to a middle seat. The check-in agent was frank enough to tell me that a PPS member had taken my original seat and could not be removed. I surmised that a late booking PPS member did not like to be seated in the middle seat, so SQ went down the list and as I was at best a *G level with AC and was on an AC reward ticket, I was selected to be bumped.