General information about SkCQ is available in the README.
When SkCQ has an unrecoverable error when processing a CL it will remove that CL from the SkCQ and post a comment that looks like: “Error when ${action}. Removing from CQ. Please ask Infra Gardener to investigate” (example).
To find the reason, search in skcq-be logs here for the CL number.
Documented here.