Index changes are dictated entirely by index rules. An index has a very clearly defined methodology and calculation method as specified by the index provider (eg FTSE/JSE, MSCI, S&P Dow Jones). There are specified rebalancing periods - FTSE/JSE indices are rebalanced quarterly in March, June, September and December.
There are various elements of the index calculation that can change, for example a company's number of shares-in-issue can change, or there can be a change in the investability of the share (how much of the company's shares trade freely on the exchange). All these changes impact the weight of the share in the index. Satrix will replicate these changes so that the funds you invest in always mimic the index you want to track.