here's my take
A good friend of mine, after 15 years with a megacorp, had a similar thing happen to her. 14 years of supervisory experience and with very good to excellent performance reviews, then the new boss from the corporate dark side showed up. da da doom. His take was that her team 'Liked her" that was not to be tolerated in the new regime. She just wasn't "tough" enough. I never saw two people so diametrically opposed; or lets say, two cats in a sack. Stressed to the point of breaking by the new overlord after 12 months of insane hours, objectives and duties; he finally dropped the PIP bomb on her. As others had stated, it's the proverbial kiss of death. The fat lady is singing , btw.
In her late-50's, she went to an attorney. No help there. He told her to sign it and she would not, there was no possible way for her to meet the PIP. He said he couldn't help her. Nice. All during the time her health was going downhill fast and once her doctor saw her and did some tests, he put her on medical leave. She was out about 7 months, all the while weighing her options and trying to get better. Worn to a frazzle.
There's a happy ending, she got a new job once she recovered a bit, better pay and saner folks to work with, in a competing firm. Now, they are kicking her old megacorp's butt in the marketplace. Turnover at the old company is at record levels and office closures are being planned, we're hearing on the grapevine. And she never did sign that PIP; though, I did see her resignation letter and it was a doozy. It started with stating that her old boss was a complete asshat and ended with "take your lousy job and shove it".
Personally, I thought it was awesome.