The claim is that given opportunity and time but mostly access (possibly remote access through backdoor or port) then it is possible for a superbly qualified person to access the program and make changes necessary to alter elections where these particular programs are used to tally votes. I forget where the specific applications are in use but I do know Mississippi and Georgia were heavily using this particular program.
Now, though, that it has been reported hopefully there will be measures taken to counter such intrusion if it can still be done. Frankly I don't know but when someone like Donald Trump is hesitant about saying he will accept any election as totally fair and accurate and getting lambasted about it I think it's unfair for there are some reasons to have concern.
It has been proven that many votes are cast under the names of people that have died or that elections have been affected by illegal votes but that goes for both Democrats and Republicans.
I'm just saying that Trump has reasons to suspect that an election might be rigged or at least has that potential to be rigged. Democrats have essentially said the same about elections that they lost so it's natural to do. So why make such a big deal when Trump says he will wait to see and make judgments then.