CSR demerged into two companies called CSR and Rinker. Removing Rinker from CSR meant that shareholders now had shares in two companies - CSR and Rinker. The price of CSR fell by the perceived value of Rinker, which was no longer in CSR, but was in the separate Rinker shares now held. Details of the demerger will be on the ASX web site under company announcements.
So-called experts will disagree on how the chart should be adjusted for this event. I have taken the view that the old data relates to a business that was significantly different to either of the demerged entities. I therefore deleted the old data and treated CSR as a new listing.
My newsletter number 33 discussed this whole area of adjusting historical data in some detail.