This situation can occur when you have more than one employer or if you had a single employer and they withheld too much. For the tax year 2018, you'll have excess Social Security withholdings if the sum of multiple employers' withholdings exceeds $7,960.80 per taxpayer.
The following TurboTax FAQ provides more detail about how to handle this situation. https://ttlc.intuit.com/replies/3300261
