I'd imagine that the fact that Willoughby had an estate worth only £700 a year, and was mixing wiht much richer people and living at a much higher rate than that, woudl be a clue that he was getting into debt. And while his differences with Mrs Smtih might nto be generally known, society wuodl know that he coudl not be sure of the favour of a capricious old lady, who might leave her money elsewhere, or might live for years...