Why does Windows update only when I'm about to present in front of 200 people?

Windows does not detect that you are about to present; update installation and restart scheduling can simply overlap that time. The practical goal is to finish pending maintenance beforehand and set the appropriate restart controls, rather than disable security updates.
Before a presentation, open Settings, Windows Update and check whether a restart is already required. Complete it while you still have time to test the presentation and conferencing setup. Under Advanced options, configure Active hours to cover your normal working period and use Schedule the restart when offered.
For a one-off event, a short update pause can be useful if available, but it is not a permanent maintenance plan or a reason to interrupt an installation already running. On an organization-managed device, restart deadlines may override user preferences; ask IT to coordinate an appropriate maintenance window. Keep a saved presentation copy and an alternative presentation device for critical events.