Sanders and his supporters were referencing the Democratic National Convention, the Democratic National Committee and the policies and procedures applied....especially the issue of "super-delegates."  I think hacked emails confirmed the system was "gamed" to advantage of the "establishment" candidate.

However, Trump is referring to the November national election.....an entirely different thing.  Such elections are controlled on a state basis and are not subject to an "overall" rigging... The only "rigging" we've seen is the systematic voter suppression perpetrated in GOP controlled states, counties and precincts.  Of course, you might reference the 2000 presidential election in Florida where the results were determined by a conservative controlled Supreme Court.

Trump is,in my opinion, realizing he's almost surely going to lose.and likely lose "big.".  He's "prepping" his most zealous followers to take actions to possibly protest his defeat. "We'd have won, but the ref's cheated us"