From the Scopus Blog:
As high content quality is very important to Scopus, and as an incentive for journals to maintain their high content quality, the title re-evaluation process has been implemented.
Six quality metrics and benchmarks have been put in place which are the basis of the re-evaluation process…These criteria have been developed with the Scopus Content Selection & Advisory Board in conjunction with our Analytical Services team.
Journals not meeting the benchmarks of all six quality metrics will be red flagged by Scopus and asked to address the issues. If the red flag remains, the journals will be reviewed by the independent Scopus Content Selection and Advisory Board (CSAB), with the possible consequence of the journal being discontinued in Scopus.
See Also: Scopus Content Policy and Selection Info