Hi Louth
It seems very reasonable to me. It sounds as if they are investigating your manager, and not you.
I think you should respect the confidentiality, at least, initially, for the reasons mentioned by Padraigb.
If you are asked for facts, give them. If you are asked for opinions, tell them that you need to reflect on the questions as you were not ready for them.
If it's about your performance, which is very unlikely, then you should simply listen but not respond. You should not walk out immediately, but you should note that you were not alerted to the nature of the meeting and will need to reflect on it.
Brendan