Sync Gateway does not do this internally.
From the log snippet it looks like this is happening as part of a BulkDocs call from a client that is doing a push replication.
Prior to the log entry posted can you see an HTTP log entry for the incoming REST API bulkdocs call, for the ADMIN API this will look like:
2017-04-21T13:08:38.849+01:00 HTTP: #001: POST /db/_bulk_docs (ADMIN)
for the PUBLIC API this will look like:
2017-04-21T13:09:33.165+01:00 HTTP: #001: POST /db/_bulk_docs (as )