I am thinking that your observation of CXA was one of...

I am thinking that your observation of CXA was one of algorithmic stock rotation as the same pattern occurred across ASX. In instance of CXA, during the day, Centre Point trades are only executed within a Centre Point pool. If you place an Order during on ASX it will not get matched on CXA. Not sure if the pools (CXA and ASX) are merged during the closing auction though. Someone else may know perhaps. To test this theory and (attempt) isolate any trade packets that may have algorithmic I plotted total IMU-ASX trades on a scatter shown below. Sell Orders are lower half and Buy Orders upper half. Note here the accumulation trades 0-052 and 0.0525. Within this range there was (near enough to) $100,000 of trades within seconds of each other with a further $200,000 on CXA. If we consider that pre-authorisation short holding packets released by a broker are generally in minimum $50,000 lots I am leaning to think it highly probable that your observation was the systematic rotation of short stock holdings. Which leaves not very much liquidity for trade for anyone else. Chart and opinions mine and mine only of course!