What do I say to a group of about 30 after I fire their manager for cause?
I just finalized my decision with HR and senior leadership Friday pm (2 days ago). I have to coordinate the day of termination with HR and security, but it will happen on the Tuesday after Thanksgiving (rough I know) which is in about 10 days. I plan to notify the manager in the morning (around 8:30) and hold an impromptu staff meeting (~10am probably). Our policy is to have security escort the person out without an opportunity to gather personal belongings. (I would prefer to let the person come back after hours to collect her belongings, but I can't.)
My first fear is actually that people will be happy and celebrate. When I took over the group, I actually received explicit requests from staff to fire the person in question. I would ask, "What can I do to help?" A couple people answered "Fire Sue" [not her real name] while about half asked for a transfer to another team. The retention data over the last two years are awful; about 10 exit interviews in the last 2 years cite my outgoing manager as the primary cause of the person choosing to leave (newsflash, I know).
Is it as simple as saying, "I know many of you were not happy with Sue's leadership, but let's remember she is a human. She has family that loves her and this will effect more than just Sue. We should not be happy that someone has lost their livelihood. If you choose to discuss this termination at all, I ask you to show sympathy."

Possibly Softer & Slower
Maybe let them digest the manager being terminated before laying out the productivity issues. It can be interpreted as 'I've fired her, and if you don't up your game, you're next'.
Have you got a transition plan to a new manager or reorg? If so, that's probably what they'll want to know right then and there. I think leaving the productivity issues for another day (soon) would serve your purposes better
There's a new Sue.
In my opinion they need to get behind their New Sue.
Deadlines are going to get missed. That's a given, in my opinion.
What was missing with Sue?
Why were deadlines being missed?
I don't think you should review expectations during this. I am sure that your team knows what the expectations are.
That should be done during quarterly performance reviews with their new manager.
I suggest the New Sue conduct one on ones and start the Trinity roll out with each team manager (if there are team managers).
https://www.manager-tools.com/2015/02/get-back-track-chapter-1-dont-mis…
https://www.manager-tools.com/2011/01/following-up