Zotero application bug report
Zotero for Mac has on several occasions today hung while annotating using the PDF reader. Restarting temporarily fixes it, but the issue recurs. The Report ID is 2045588702, and the Debug ID is D379931359.
https://www.zotero.org/support/debug_output#logging_to_a_terminal_window
It's backing up the database — possibly to do an update, though I'm not sure why if you've been running it regularly — and may take a short while if the file is extremely large or if there's a lot of other activity on your computer.