Why would the IRS want me to send supporting docs for my submitted Federal 1040 Schedule 1 line 22 amount ?

https://www.irs.gov/pub/irs-pdf/f1040s1.pdf - That line is the sum of your additional income. Which of the preceding lines did you claim?

Capital gains, line 13, was the only item entered for Additional Income. It was an unusually large amount of my income for 2018. 

Did you receive a 1099-B for all the capital gains transactions and was the cost basis shown on the 1099-B for all of them? If not, the IRS may be looking for support for the cost basis you claimed on your tax return.  Was the request you received specific about what they wanted?