I had a similar situation many years ago, but no signing bonus. Company-wide, I asked my boss about it, she said she wasn't signing it either. I felt comfortable that there would be no targeted retaliation in my chain of command.
I wouldn't worry about it if you don't want to sign it, it's just an HR fishing expedition to try to mitigate future risk (you can bet it will be mandatory for new-hires though). Surprised they're just doing this now, it was 20 years ago for me.
In fact I personally might take the bonus unless I had reason to suspect that I might have to litigate something, or I might be around for many years (things can change over time).