It's an informed assumption based on historical precedent and the combination of plausible deniability with level of ease to implement that only statistics can really disprove as anything but speculation. There's also less eyes this time around because independent observers like OSCE have been prevented from participating.

These issues are nothing new, but are so old some of the articles that have discussed them in the past are no longer even accessible:

https://verifiedvoting.org/vote-flipping-and-touch-screen-calibration/