Leibniz's Principle of Sufficient Reason (PSR) states that: for every fact F, there must be a sufficient reason why F is the case (https://plato.stanford.edu/entries/sufficient-reason/#WhatSuffReas). This applies to all contingent facts.
For Leibniz, a proposition is contingently true iff it is true in this world and false in another world. And a proposition is necessarily true iff it is true in every possible world. (https://plato.stanford.edu/entries/leibniz-modal/#NatMod)
However, if we define a fact to be necessary iff it cannot be otherwise, then doesn't the PSR lead us to conclude that all contingent facts are necessary? If for every contingent fact F, there is a sufficient reason F', which may itself be contingent, but then would be in turn grounded by another sufficient reason F'' ad infinitum, then wouldn't we be pressed to say that in our world (at least) the contingent fact is necessary in that it couldn't be otherwise?