It really is nothing to be suspicious about. Full vote by mail had already been the norm in some US states for years, and most states allowed for it in specific circumstances. The infrastructure, laws, etc., were already there, they just needed to be expanded. Expanding it has always been in the national conversation, it has just been a matter of figuring it out and priority.

So when a global pandemic occurs and we're trying everything we can to isolate and socially distance, that priority changes real quick. People get talking and problems get solved.

Of course, sore losers will complain about anything to justify their loss, and this "new thing" was a prime scapegoat. It was also well known ahead of time that the mail in votes would be largely Democratic (because COVID was VERY politicized and democrats were more likely to follow quarantine guidance and therefore vote by mail). So when the votes came in, they pointed to that imbalance and called it "fraud".

Besides all that, there's no reason to be more suspicious of mail-in ballots than in-person ones. In-person, you mark a paper ballot and then put it in a stack... which then gets mailed somewhere else. If someone is going to be changing mail-in ballots, then they're already in a position to be changing regular ones as well (and every election security professional will tell you that paper ballots are more secure than electronic ones).