Axios: "Why it matters: Democrats are exponentially more likely to vote by mail than Republicans this year — and if enough mail-in ballots are lost, rejected on a technicality or undercounted, it could change the outcome of the presidential election or other key races."
Those problems antedate conspiracy theories du jour and Trump's anxieties. Mailed ballots are more at risk of being mislaid than in-person ballots, and mailed ballots typically are rejected at higher rates (for errors) than in-person ballots.
That a few sacks of mail were found discarded and a handful of mailed ballots found discarded are objectively miniscule faults, but subjectively such incidents have a large impact on perceived (output) legitimacy. Perceived legitimacy - trust - counts more than the objective measures.
There is also a custodial problem which can't be wished away. The chain of custody for "shotgun" mail balloting is weaker than that of on-demand mail balloting, which in turn is weaker than that of in-person voting. Repeated reports of the numbers of people improperly retained on registered voter lists, coupled with a tendency by Democrats to resist cleaning up voter rolls, weakens perceived legitimacy. For that, Democrats have no-one but themselves to blame. Those who want to increase the mechanisms of voting have the most interest in reassuring voters that the mechanisms are tamper-resistant - not by words, but by deeds - but they will not do so.