That's not exactly true. It is possible to lose an election despite rigging it. Given the U.S elections doesn't have a 100% turnout, we could conceive a scenario where the riggers were expecting a much lower turnout and added as many votes as they thought they could without it becoming too suspicious. Their sad luck being that a much greater turnout against their vote actually happened.

Of course, the burden of proof lies on those making the accusation of rigging. And seeing as these accusations allege the rigging were made in a station-by-station basis, that'll be a lot of work!

Florida election supervisors were accused of filling in blank election ballots, i.e. rigging the votes.

http://ijr.com/opinion/2016/11/261634-florida-voter-fraud-broward-county/

There has also been a issue of the voting machines registering votes for Trump as votes for Hillary.

http://www.breitbart.com/live/2016-election-day-live-updates/officials-pa-voting-machines-incorrectly-registered-straight-republican-votes/

Do note that the person who owns the company which makes the voting machines, George Soros, is one of Hillary’s biggest donors.

So yeah, conflict of interest at the very least.