Babel seems to autocreate a category sometimes, and to make the edit for that, it also needs to ensure its system user exists, which is also autocreated.
All this can be on GET from api.php?action=parse. And even in cases where an autocreation isn't needed (i.e. the system user already exists), User::newSystemUser() still opens a master connection.
Expectation (masterConns <= 0) by ApiMain::setRequestExpectations not met: [connect to 10.64.16.30 (metawiki)] #0 /srv/mediawiki/php-1.29.0-wmf.6/includes/libs/rdbms/TransactionProfiler.php(160): TransactionProfiler->reportExpectationViolated() #1 /srv/mediawiki/php-1.29.0-wmf.6/includes/libs/rdbms/loadbalancer/LoadBalancer.php(587): TransactionProfiler->recordConnection() #2 /srv/mediawiki/php-1.29.0-wmf.6/includes/GlobalFunctions.php(3077): LoadBalancer->getConnection() #3 /srv/mediawiki/php-1.29.0-wmf.6/includes/user/User.php(671): wfGetDB() #4 /srv/mediawiki/php-1.29.0-wmf.6/extensions/Babel/BabelAutoCreate.class.php(96): User::newSystemUser() #5 /srv/mediawiki/php-1.29.0-wmf.6/extensions/Babel/BabelAutoCreate.class.php(54): BabelAutoCreate::user() #6 /srv/mediawiki/php-1.29.0-wmf.6/extensions/Babel/BabelBox/LanguageBabelBox.php(183): BabelAutoCreate::create() #7 /srv/mediawiki/php-1.29.0-wmf.6/extensions/Babel/Babel.class.php(192): MediaWiki\Babel\BabelBox\LanguageBabelBox->getCategories() #8 /srv/mediawiki/php-1.29.0-wmf.6/extensions/Babel/Babel.class.php(119): Babel::mGenerateContent() #9 /srv/mediawiki/php-1.29.0-wmf.6/extensions/Babel/Babel.class.php(47): Babel::mGenerateContentTower() #10 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Parser.php(3379): Babel::Render() #11 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Parser.php(3103): Parser->callParserFunction() #12 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Preprocessor_Hash.php(1015): Parser->braceSubstitution() #13 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Preprocessor_Hash.php(1448): PPFrame_Hash->expand() #14 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Parser.php(3253): PPTemplateFrame_Hash->cachedExpand() #15 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Preprocessor_Hash.php(1015): Parser->braceSubstitution() #16 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Parser.php(2917): PPFrame_Hash->expand() #17 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Parser.php(1265): Parser->replaceVariables() #18 /srv/mediawiki/php-1.29.0-wmf.6/includes/parser/Parser.php(441): Parser->internalParse() #19 /srv/mediawiki/php-1.29.0-wmf.6/includes/StubObject.php(113): Parser->parse() #20 /srv/mediawiki/php-1.29.0-wmf.6/includes/StubObject.php(139): StubObject->_call() #21 /srv/mediawiki/php-1.29.0-wmf.6/includes/content/WikitextContent.php(330): StubObject->__call() #22 /srv/mediawiki/php-1.29.0-wmf.6/includes/content/AbstractContent.php(497): WikitextContent->fillParserOutput() #23 /srv/mediawiki/php-1.29.0-wmf.6/includes/api/ApiParse.php(274): AbstractContent->getParserOutput() #24 /srv/mediawiki/php-1.29.0-wmf.6/includes/api/ApiMain.php(1532): ApiParse->execute() #25 /srv/mediawiki/php-1.29.0-wmf.6/includes/api/ApiMain.php(541): ApiMain->executeAction() #26 /srv/mediawiki/php-1.29.0-wmf.6/includes/api/ApiMain.php(512): ApiMain->executeActionWithErrorHandling() #27 /srv/mediawiki/php-1.29.0-wmf.6/api.php(83): ApiMain->execute() #28 /srv/mediawiki/w/api.php(3): include() #29 {main}