This happens when you're trying to deploy an app to root context (either using the name ROOT.war or "/" context in jboss-web.xml),

but the AS welcome page is already occupying that.

The AS CLI command to disable default root context is:

[standalone@localhost:9999 /] /subsystem=web/virtual-server=default-host:write-attribute(name=enable-welcome-root,value=false)   
{
    "outcome" => "success",
    "response-headers" => {
        "operation-requires-reload" => true,
        "process-state" => "reload-required"
    }
}

Then, restart the AS (or at least web subsystem).

Or, shut down the server and edit the standalone.xml:

<subsystem xmlns="urn:jboss:domain:web:1.2" default-virtual-server="default-host" native="false">
    <virtual-server name="default-host" enable-welcome-root="false">
    ...
</subsystem>