just a wild guess, but maybee your head down caused a dip in your shoulder that makes you come a bit more in to out maybe.....

there are better experts on this site that can maybe explain the swing better and why you are getting the results you describe........

but I think keeping the HEAD down is bad..... it causes you to dip your shoulder in the dowswing..... you want your head to be inline with your spine angle, and keep your EYES down on the ball. that way you can still see what you are hitting and have you head clear of your shoulders in the downswing.