Because global indices cover a wider range of companies than individual asset trades, there are several factors that can affect their performance. For example, commodity prices can have a huge impact on exchanges such as the FTSE 100, which has a large proportion of commodity stocks.
Then there are changes in a company's structure or composition, which can affect its market capitalization or share price and subsequently affect the performance of that index.
Traders will also keep a close eye on the financial results of companies that may have a direct impact on their stock prices. Economic and geopolitical events are also likely to play a role in the performance of some global indices.