Why does an off-center cargo load that makes a trailer lean increase rollover risk in a combination vehicle?

AAMVA CDL Manual, Section 6.1.1 'Rollover Risks,' p. 6-1

Even a properly loaded trailer can become dangerous if the weight shifts to one side during loading or in transit, since a leaning trailer is already partway to tipping over. Checking that cargo is centered and spread out is a quick pre-trip step that directly reduces rollover risk.