This is by far the largest increase since the 3.6% COLA that was declared 2012 and the second largest since 2009. COLA increases are related to inflation in the economy, as measured by the CPI-W: the Consumer Price Index for Urban Wage Earners and Clerical Workers.
The Contribution and Benefit Base, also known as the “wage base,” will increase from $128,400 to $132,900 in 2019. This means that an individual’s earnings up to $132,900 will be subject to Social Security taxes next year. Changes in the wage base are related to increases in the National Average Wage Index.
Changes in other inflation-sensitive numbers for 2019, such as the exempt amount for the Earnings Test, have been announced as well. See https://www.ssa.gov/news/press/factsheets/colafacts2019.pdf