The only thing I can figure is, they picked Bernie to run against Hillary just to try and make it look legit but never figured on Bernie outdoing the future convict. It all has backfired and they are having to resort to ‘in your face’ trickery to force the outcome to Hillary.
Bingo. Bernie was just supposed to be a foil so the DEMe candidate could be shown "winning" the primary. It would provide zero publicity if Hillary were just handed the nomination and it would puncture the illusion the voters control the process.
However, the DEMe has now lost control of the process and it's not going according to plan. Much like the GOPe has lost control and they're preselected candidate Jeb(!) is now polling in single digits. The whole GOPe and DEMe control structure is in disarray and in danger of collapsing. Good.