It's a feature for selected object, if you don't need it please don't press Tab key when you select objects. you can click the empty area to deselect objects.

I am not trying to select or deselect an object. I am selecting an object, then hitting 'tab' to change its attributes - name, prefix, etc. It recently started happening, so I don't think it is an update, but maybe something I accidentally turned on. Is there a setting that causes this? Any other ideas on what is causing it?

No, it is a new feature for v5.9

It may be a useful feature, but it is a nuisance for the most part (being activated by the tab key). Is there another keystroke to select the first text field of the attributes sidebar that won't bring up this dialog? Maybe you could change the keystroke to activate this dialog to shift-tab in a future release. This feature is a fairly big inconvenience for my usage.