Simply put, reverse onus is a rule of evidence. Normally, if you're alleging something, you have to prove it. That makes legal sense in most circumstances.
In this context, when we're often dealing with a power imbalance, the person who is blowing the whistle is usually not the person who has the power and the control. Often they are excluded from the workplace after this, so they don't have the evidence. It is a very difficult evidentiary burden to prove. Often it gets to a fifty-fifty, a “he said, he said” or a “he said, she said”, and that weighs in favour of the respondent in those cases. Reverse onus would change that balance—very slightly, in fact—so that the employer, who has the control, who has the power, and who has the other employees at their disposal, bears a bit more of an onus.
It's not unheard of. We have managerial exclusions, for example, which depend on whether an employee falls within a union or not. For certain exclusions, the onus is on the union and for certain exclusions the onus is on the employer. It's not unusual. It's not typical, but it's a well-known legal principle, and it serves a purpose in this case.