There were approximately 1 million SGH shares traded on the ASX by 3:59pm yesterday. Between 4-4:10pm in the 'Pre-CSPA' there were 3.7 million (approx) shares matched. If you notice at 4:41pm & 5:03pm in the (Adjust/overnight trading) a further 489k of shares were exchanged. This exchange was a portfolio special crossing (SX).
The huge volume was due to the S&P/ASX quarterly rebalance.