According to the documentation a thread dump can be initiated by sending a SIGQUIT to the jvm.
So it may be that your shutdown procedure actually triggers a thread dump - and looking from an operational perspective it does not hurt to do it yet helps a lot in cases where there are problems.
You do not report problems with Tomcat or the application themselves, just asking why you get the thread dump? I'd guess there is something in the shutdown script, so you may want to check the systemd configuration.
Otherwise the webapp itself could also trigger the command and you might want to test your tomcat without a webapp installed.
See also: Taking thread dumps in production