I'm guessing your Disqus widget appends data on the bottom of your page.
Only then is your page long enough to actually scroll to your anchor.
One way to solve your problem would be to attach a JavaScript handler (callback) to the load function of your comment section which would be triggered as soon as the AJAX requests are finished.
In that callback function you can trigger the scroll to your anchor like this:
window.location.hash = '#anchor';