I'm just speculating - it may be a disability or health issue that has come to light and made them go "uh oh, there could be a discrimination claim here". Or maybe the employee has something on the company that they don't want made public.
In my own case, I have both disabilities and 'something on the company'.
If your employee does have a disability or health concern, maybe they disclosed to HR and HR did not share it with you. That's not HR's fault, its the law on data protection etc.
You should still always consult with HR before instigating PIP anyway and whilst HR may not be able to go into specific details, they may be able to offer some advice. So that could be a procedural issue that can lead to it being over turned.
Also, some individuals don't see themselves as having a disability or if they do, may not realise their rights until someone points it out to them - this could be the case here.
eg cancer (even once recovered), severe allergies, severe asthma, depression can all be covered by the equality act but not everyone realises this. So it is possible that either the employee or someone higher up has discovered something?
I'm just speculating though. There could be any number of reasons for it being overturned but I wouldn't assume that it is an unfair or wrong reason. If they won't tell you the reason then my bet is that its either discrimination or 'something on the company'.
Oh....or related/married to someone high up.