Well, to be fair, given all the gerrymandering in the US the system is pretty rigged in multiple ways anyway. But in this particular case Trump is going on about postal voting, which is likely to be a factor in an election affected by Covid-19, while strangely being supportive of absentee ballots, which are in practice the exact same thing.
Recently, Trump has appointed a new postmaster general, Louis DeJoy, who has overseen a major restructuring of the postal service that is likely to have the effect of increasing delivery times, which in turn could affect postal voting if they have to arrive at the counting office by election day. Whether this is a coincidence or deliberate I don't know, but it's difficult not to see a link between a President railing against postal voting, and the same president appointing somebody taking action that will affect postal voting.