Don't you see any other variables causing debt and equity to change? They are not only interdependent securities. In 2008, equity market itself was in harm. Do not consider equity as an only alternative source for debt and vise versa. The rates on US Treasury securities were volatile doesn't at all suggest that its alternative investment source were not volatile. So how could one move his funds to equity when equity market itself was fluctuating along with real estate and commodities.
This was in response to
And fluctuation in rates always works in favor of equity.
You missed the point. The above is true only holding everything else constant.