I am going to guess based on my low opinion of all political parties. I am guessing that the Republican party thinks that the people who are not going in to the polls to vote in person (drive through and mail in ballots) are primarily doing so due to concerns about COVID. Again, as a general rule the party believes that those people concerned about COVID are primarily Democrats. Even when they are not Democrats, the party thinks those with concerns will vote for Biden over Trump. If you believe the polls, this last is probably a valid belief. Either way, this means that the Republican party believes that any mail-in or drive-thru ballots thrown out will be more Biden votes than Trump votes. Throwing them out helps increase the Trump win. Thus, the move to throw the votes out.
While i tend to agree with the basic premises I listed, i don't agree with throwing out the votes. If Trump cannot win the election honestly, he does not deserve to stay in office, IMO.