Well if your've varnaished the shelves, friction will be reduced. And if the shelves angle of pissedness is steep and close to the slamming door, (Which in that small little basement, it would be) the chances that that jar/glass whatever will jump/slide far enough for the energy of the objects movement will overcome the resisting force that the varnishedness will allow..
He could have been quite quite fastidious yet surreal/eccentric in his choice of shelving angles..
An individual who is able to keep half a football team of people locked in a cellar all those years without getting caught, is likely to have a taste for finishing the job's little details (although this does, I admit, contrast greatly with the theory that he didnt employ usage of a spirit level-quite the anomally).
(Unless he is a jailer who knows that incarceration in an environment with wonky, heavliy painted shelves with varnish on them will unsettle those incarcerated. Cause lets face it, it'd bug the shit out of you or I.)
It also has the effect, I should imagine, that once the incarcerated are released back into the outside world, after many years of wonky shelvedness, that properly angled shelves that they would soon experience might actually cause them to freak out.
Pure evil.