In Georgia it is generally illegal to gather up and deliver ballots from other people. I'm not a lawyer, or a Georgia resident but many states have laws like that. Usually it is legal to deliver an absentee ballot from a family member, or something like that but illegal to gather up a lot of ballots and deliver them unless you are a post office employee.
In the investigation by True The Vote they have identified people that appear to have driven over a large area and visited multiple ballot drop box locations. The question is why? It might be legitimate if it turned out the person was an election official delivering ballots from the boxes to the election department's headquarters.
But it is more likely that the person making multiple stops at ballot drop boxes was either harvesting ballots and then dropping them off, or doing something else they shouldn't have been doing, like voting multiple times.
Based on other things True The Vote has said it appears that the people they tracked were harvesting ballots, which is illegal in Georgia.
Ballot harvesting is illegal in many states because ballot harvesters tend to be a source of corruption. For example, often they are paid a fee for every ballot they collect that supports a particular candidate. Sometimes ballot harvesters improperly influence voters in the process of collecting ballots from them.
And videos show that some of the people visiting multiple drop boxes multiple times were removing batches of ballots that had been stuffed into backpacks. Some were wearing rubber gloves.