How do I explain a forced resignation in a job interview?

I have been a stellar employee for 8.5 years. In fact last year my bonuse was higher than it has ever been. I am 55. I also had a workers compensation claim last year. Out of the blue I was put on probation. I had several things they wanted me to do so I did them. When we met three months later they asked me to resign even though I had completed all of the tasks requested during the probation. They did offer me a nice severance package. But now I am struggling with how to answer why I left my last job. My boss would not tell me why. I still do not know why. So I cannot honestly answer why I left my last job. To me it was better to sign the agreement and leave with a nice severance and options to be rehired than to be fired so I did it. I just don't know how to answer the question, "Why did you leave?" because I honestly do not know and will likely never know. What would you suggest?

Thank You,
Jobless without a reason