To bootstrap the network early on, the Synthetix team implemented a high incentive inflation model from March 2019, designed to attract capital and promote staking. More on Synthetix's monetary policy and the decision-making process here.
This inflationary monetary policy was planned to begin as a 75% annualized rate distributed weekly, and was set to halve each year for five years. However, a community-driven initiative that reached consensus through community governance smoothed the inflation rate in December 2019. You can read a detailed post about this process here.
This update also added a perpetual 2.5% annual inflation beginning on September 7, 2023, the 235th week on the SNX inflation schedule. This was determined through the same comumunity governance process.