While there may not be anything inherently bad about debt levels, the level of debt fundamentally affects stock price by the equation of total enterprise value = stock + debt - cash (i.e. total enterprise value is the amount someone would need to pay to buy all the outstanding equity AND take on or retire the outstanding debt). So if there is hidden debt that isn't being factored in by investors in means the stock price should go down because "how much a company is worth" (i.e total enterprise value) would remain the same, it's just now that more is debt and less is shareholder equity.