Why sendMessage api in content_script fires before page url is loaded?

Question

In content_script, I have used chrome's sendMessage api to exchange data with background script. But using sendMessage api the script outputs unexpected results.

As I start typing the url, in a new-tab, (url is not yet loaded) the sendMessage api fires to the background script (I checked it by placing debugger at background script).

I tried following cases in my content.js file to understand the problem:

Case1: Here, as url is typed (not loaded) this api is fired. (unexpected)

chrome.runtime.sendMessage({source:'registerTab', data:''}, function(){
console.log("response_after_background_page")
});

Case2: Here, it wait for the url to load and then there is an alert message. After that the sendMessage api fires. (this result is expected in case1)

alert();

chrome.runtime.sendMessage({source:'registerTab', data:''}, function(){
    console.log("response_after_background_page")
});

Case2: This outputs again as case1 and api is fired even before the url is loaded. (unexpected result as in case1)

console.log("SOme_message");

chrome.runtime.sendMessage({source:'registerTab', data:''}, function(){
    console.log("response_after_background_page")
});

I have tried excluding new-tab url in manifest.json file:

  "content_scripts": [
    {
      "matches": ["*://*.google.co.in/*", "*://*.somehost.com/*"],
      "exclude_matches": ["*://*/_/chrome/newtab*"],
      "js": ["libs/jquery-1.12.3.min.js", "libs/underscore-min.js", "content.bundle.js"],
      "run_at": "document_end"
    }
  ],

I expect my url to load and then only content.js should be injected. And then only the sendMessage api should fire. Where am I getting wrong?

Edit:

I am building a extension which tracks webpage whose url matches as rule specified in manifest.json file. And when the matched url is loaded the content script should fire a sendMessage call to background page to keep record of this created page. But as sendMessage api is called before the url is loaded some junk url info is sent to background page. I want all parts of content_script to load only after the url is fully loaded not when it is being typed.