@DavidR,
Why else would Avast want me to restart my computer?
Here we go again, zero information, a screenshot would be most helpful.
I didn't get any warning or notifications to restart (as I have said program set to manual update), only when I manually checked did it update and then I restarted.
You should have checked what program update you already had when this occurred, trying to find out after the fact makes it impossible to help. Nor do you say what the program version is that requires a restart.
I suspect the opening of the avast UI may (and that's a big MAY) have triggered the check, but there really is no information to work with.