Often a PE buyout is a bankruptcy, or a last-ditch alternative to one. Breaking the company down for parts is often what the creditors want, explicitly or otherwise, and the PE firm is just being paid to execute it. (There are definitely cases where one creditor arranges things to screw over another - but again I think blaming the PE firm for that is misplaced.)

They have their share of failures like any other industry, but IME Hanlon's Razor - or just legitimate risk-taking that doesn't pan out - applies to most PE bankruptcies.